Photomask and methods for manufacturing and correcting photomask

ABSTRACT

The present invention provides a halftone mask comprising an assist pattern and a manufacturing method of the halftone mask, which uses an ArF excimer laser as an exposing source, is used for a projection exposure by an off axis illumination, does not resolve the assist pattern while keeping the focal depth magnification effect as the assist pattern, and may form a transferred image having high contrast of a main pattern. A photomask is a photomask comprising the main pattern which is transferred to a transfer-target surface by the projection exposure and the assist pattern which is formed nearby the main pattern and not transferred, characterized in that the main pattern and the assist pattern are each constituted from a semi-transparent film made of the same material, a retardation of 180° is generated between the light transmitting through the main pattern and the light transmitting through a transparent region of a transparent substrate, and a predetermined retardation within the scope of 70° to 115° is generated between the light transmitting through the assist pattern and the light transmitting through the transparent region of the transparent substrate.

TECHNICAL FIELD

The present invention relates to a photomask and a manufacturing methodof the photomask, which is used for a photolithographic technique usinga shortwave exposing source such as an excimer laser exposure deviceused for patterning a semiconductor device, and a correcting method ofthe photomask and the corrected photomask, and particularly relates to ahalftone photomask and a manufacturing method of the halftone photomask,in which an assist pattern is disposed nearby a main pattern, and acorrecting method of the photomask and the corrected photomask.

BACKGROUND ART

In order to realize high integration and ultramicronization of asemiconductor device developing from a half pitch of 65 nm into 45 nmand further 32 nm, a high-NA technique for increasing the numericaperture of a projector lens, an immersion exposure technique forexposing while making a high-refraction medium intervene between aprojector lens and an exposure target, and an off axisillumination-mounted exposure technique have been put to practical useas a high-resolution technique in an exposure device inphotolithography.

Phase shifting masks such as a Levenson (also referred to asShibuya-Levenson) phase shifting mask for improving resolution by aphase shifting effect utilizing optical interference, a halftone phaseshifting mask (simply referred to as a halftone mask hereinafter)constituted from a part for transmitting light and a part forsemi-transmitting light, and a chromeless phase shifting mask includingno light shielding layers such as chrome have been used as measures toimprove resolution in a photomask (also referred to as a maskhereinafter) used for photolithography, together with micronization andhigh precision of a conventional binary mask constituted from a part fortransmitting light and a part for shielding light.

In a photolithographic technique, the minimum dimension (resolution)transferable by a projection exposure device is in proportion to awavelength of light used for exposure and is in inverse proportion tothe numeric aperture (NA) of a lens in a projection optical system, sothat shorter wavelength of exposing light and higher NA of a projectionoptical system have been progressing in accordance with a request formicronization of a semiconductor device; however, it has become a limitto make only shorter wavelength and higher NA satisfy this request.

Thus, a super-resolution technique for intending micronization bydecreasing a value of process constant k₁ (k₁=resolution linewidth×␣numeric aperture of projection optical system/wavelength ofexposing light) has been proposed for improving resolution in recentyears. Methods called a method of optimizing a mask pattern by providingan assist pattern and a line width offset for the mask pattern inaccordance with the properties of an exposure optical system, or amethod by an off axis illumination (also referred to as anoblique-incidence illumination method) are offered as such asuper-resolution technique. An annular illumination (also referred to asAnnular) using a pupil filter, a dipolar illumination using a dipolar(also referred to as Dipole) pupil filter, and a quadrupole illuminationusing a quadrupole (also referred to as Cquad) pupil filter areordinarily used for projection exposure by an off axis illumination.

The method using an assist pattern is a lithographic method using aphotomask having the effect of improving resolution and focal depth of amain pattern by disposing a pattern (referred to as an assist patternhereinafter), which is a resolution limit or less of a projectionoptical system and not transferred on a wafer, nearby a pattern(referred to as a main pattern hereinafter), which is transferred on awafer (for example, refer to Patent Literature 1). The assist pattern isalso called SRAF (Sub Resolution Assist Feature) (the assist pattern isalso referred to as SRAF hereinafter in the present invention).

However, in accordance with micronization of a semiconductor devicepattern, a photomask having an assist pattern has had difficulty inproducing the mask. First, as described above, the difficulty isconceived to be such that the assist pattern itself needs not to beimaged on a wafer and needs to be minuter in dimension than a mainpattern. As a result, in accordance with micronization of the mainpattern dimension, the line width dimension of the assist pattern to berequested is micronized from several hundreds nm to such a minuterdimension as to be approaching a limit in producing. For example, in thecase of forming a semiconductor device of a 65-nm line width on a wafer,the line width dimension of the main pattern on the mask (a reticle withan ordinary tetraploid pattern) is formed into approximately 200 nm to400 nm in addition to optical proximity correction (OPC), while the linewidth dimension of the assist pattern becomes 120 nm or less and themask production becomes extremely difficult. As described above, thedimension of the assist pattern is a great problem in producing the maskon the exposure conditions of transferring a pattern with a half pitchof 65 nm or less.

In addition, with regard to transfer properties of the mask on which apattern with a half pitch of 65 nm or less is transferred, as describedlater, a halftone mask allows more favorable transferred image than abinary mask so frequently that it is greatly desired that the maskhaving the assist pattern is constituted into a halftone mask and ahalftone mask having the assist pattern is also proposed (for example,refer to Patent Literature 2, Patent Literature 3 and Non PatentLiterature 1). However, a halftone mask ordinarily has a minus bias inthe mask pattern dimension by reason of transfer properties, so that itis requested that the dimension of the assist pattern formed from asemi-transparent film as a halftone mask is smaller than the dimensionof the assist pattern of a binary mask formed from only a lightshielding film. In a generation from 45 nm to 32 nm of a half pitch of asemiconductor device, the assist pattern dimension of 60 nm or less inmask line width has been requested, depending on the design and exposureconditions of a semiconductor.

Also, in accordance with micronization of the assist pattern, in a maskproduction process such as washing, or in the case of rewashing a maskwhich became dirty during using in an exposure device, with regard to ahalftone mask including a conventional assist pattern, an aspect ratio(pattern height/pattern width) of the assist pattern approaches 1 andthe problem is to cause phenomena such that part of the assist patternis chipped, the assist pattern is peeled off a substrate surface, andthe assist pattern falls in the line width direction.

A photomask, in which a retardation of 180° is generated between thelight transmitting through a semi-transparent pattern and the lighttransmitting through a transparent region of a transparent substrate, apredetermined retardation within the scope of 50° or less is generatedbetween the light transmitting through a semi-transparent assist patternand the light transmitting through a transparent region of a transparentsubstrate, and focus properties of the semi-transparent pattern areflattened, is proposed in Patent Literature 2 as correspondence tomicronization of the assist pattern by a halftone mask. FIGS. 24A and24B are a plan view (FIG. 24A) and a longitudinal cross-sectional view(FIG. 24B) of a photomask described in Patent Literature 2. Thephotomask according to Patent Literature 2 allows an assist patternprovided nearby a line pattern as a main pattern to be also formed intothe same dimension as the main pattern.

The halftone mask having the assist pattern described in PatentLiterature 2, as shown in FIGS. 24A and 24B, is a mask such that asemi-transparent pattern as a main pattern 1 is a line pattern with aline width of 0.3 μm on a wafer and a semi-transparent assist pattern 2is a line pattern with the same line width as the main pattern 1 on theright and left thereof. In this mask, the main pattern 1 has a two-layerconstitution such that a transparent film 304 is further formed on asemi-transparent film 302, a retardation of 180° is generated betweenthe light transmitting through the semi-transparent main pattern 1formed from the two-layer film and the light transmitting through atransparent region of a transparent substrate 301, on the other hand, apredetermined retardation within the scope of 50° or less is generatedbetween the light transmitting through the semi-transparent assistpattern 2 and the light transmitting through a transparent region of thetransparent substrate 301, and focus properties of the semi-transparentpattern are flattened.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Application Publication No.    H07-140639-   Patent Literature 2: Japanese Patent No. 2953406-   Patent Literature 3: Japanese Patent Application Publication No.    2003-302739

Non Patent Literature

-   Non Patent Literature 1: N. V. Lafferty, et al., Proc. of SPIE Vol.    5377, 381-392 (2004)

SUMMARY OF INVENTION Technical Problem

However, the halftone mask having the assist pattern described in PatentLiterature 2 is a mask of a generation intended for a semiconductordevice in which an i line (365 nm) of a mercury-vapor lamp or a KrFexcimer laser (248 nm) is used for an exposing source, the numericaperture NA of a projection optical system is as small as 0.6, and thepattern dimension on a wafer is of a submicronic order of 0.3 to 0.35μm; in the case of being used as a mask for a semiconductor device inwhich an ArF excimer laser presently proceeding toward practical use isused as an exposing source, an exposure device such that NA is as highas 1 or more, desirably approximately 1.3 to 1.35 is used, and thepattern dimension on a wafer is a half pitch of 65 nm or less, further45 nm and 32 nm, the problem is as follows.

That is to say, according as process constant k₁ becomes small, an offaxis illumination is used for improving resolution of the main pattern,and the problem is that the assist pattern is easily resolvedaccordingly. In addition, the problem is caused such that the assistpattern is easily resolved on a transfer-target surface by reason of asteric effect by the thickness of a mask in a vertical direction to themask substrate plane (a three-dimensional effect of a mask) throughoblique-incidence irradiation of an off axis illumination. With regardto the halftone mask having the assist pattern described in PatentLiterature 2, even though retardation of the main pattern is within apredetermined scope, the assist pattern is resolved by thethree-dimensional effect, and change in dimension becomes asymmetricwith respect to defocus; the problem is caused such that the quality ofa transferred image deteriorates to be inappropriate for practical use.

Also, with regard to any of the photomasks described in PatentLiterature 2, Patent Literature 3 and Non Patent Literature 1, only themain pattern has a two-layer constitution such that a semi-transparentfilm is superposed on a lower layer of a transparent substrate side, anda light shielding film or a semi-transparent film or a transparent filmmade of a material different from the lower layer is superposed on anupper layer thereof. In the production of a photomask having the assistpattern of a semi-transparent film, a film-forming process of the mainpattern needs to be performed twice and the problem is that a productionprocess becomes complicated. In addition, in the production of aphotomask described in Patent Literature 2, the problem is that thealignment of a first pattern formed on a transparent substrate with asecond pattern formed subsequently becomes difficult in accordance withpattern micronization, a space between the main pattern and the assistpattern needs to be determined at a value in consideration ofmisalignment (ordinarily, approximately 200 nm) or more, and theequalization of the assist pattern width with the main pattern widthbecomes difficult in accordance with pattern micronization.

As described above, although the halftone mask including the assistpattern has been greatly requested in accordance with micronization of asemiconductor device pattern, a photomask including a conventionalassist pattern does not correspond to micronization as a mask for asemiconductor device with a half pitch of 65 nm or less, further 45 nmand 32 nm, and the problem is that the production thereof becomesdifficult.

Thus, the present invention has been made in view of the above-mentionedproblems. That is to say, a first object of the present invention is toprovide a halftone mask having an assist pattern and a manufacturingmethod thereof, which mask uses an ArF excimer laser as an exposingsource, is a mask used for a projection exposure by an off axisillumination, does not resolve the assist pattern while keeping thefocal depth magnification effect as the assist pattern, restrainschipping and falling of the assist pattern, and may form a transferredimage having high contrast of a main pattern.

Also, as described above, in accordance with micronization of the assistpattern, in a mask production process such as washing, or in the case ofrewashing a mask which became dirty during using in an exposure device,the problem is that part of the assist pattern is chipped.

Thus, the present invention has been made in view of the above-mentionedproblems. That is to say, a second object of the present invention is toprovide a halftone mask having an assist pattern, which mask restrainschipping of the assist pattern from occurring.

Originally, the assist pattern is designed as a mask so as not to beresolved on a transfer-target surface, so that all of the plural assistpatterns are not necessarily transferred; yet, the problem is causedsuch that part of the assist pattern, for example, the assist patternnearby a main pattern or part of the nearby assist pattern is resolved.

Various factors such as the case where the precision of mask designsoftware is so poor as to bring imperfection in a mask design stage, thecase where the error of the assist pattern dimension actually producedis large, the case where an optical system of a semiconductor exposuredevice has a subtle error, and the problems in the properties of asemiconductor resist are conceived as the cause for the above-mentionedassist pattern or part thereof to be resolved on a transfer-targetsurface.

As described above, although the photomask including the assist patternhas been greatly requested, pattern transfer is performed in a regionclose to the limit of a minute pattern in accordance with micronizationof a semiconductor device pattern, so that the problem is caused suchthat the assist pattern is resolved on a transfer-target surface afterexposure transfer; even in attempting not to be resolved by correctingthe assist pattern line width to a smaller value for correcting the maskof the assist pattern to be transferred, it is difficult to correct theassist pattern of, for example, 60 nm or less in mask line width to asmaller width, and the problem is that the photomask needs to beproduced again.

Thus, the present invention has been made in view of the above-mentionedproblems. That is to say, a third object of the present invention is toprovide a correcting method of a photomask and the corrected photomask,which method corrects an assist pattern by a sure and comparatively easymethod with regard to the photomask in the case where the assist patternis resolved on a transfer-target surface in the photomask which uses anArF excimer laser as an exposing source and has the assist pattern usedfor a projection exposure by an off axis illumination.

Solution to Problem

To solve the above-mentioned problems, the present invention provides aphotomask using an ArF excimer laser as an exposing source, being usedfor a projection exposure by an off axis illumination, and comprising ona principal plane of a transparent substrate a main pattern transferredto a transfer-target surface by the projection exposure and an assistpattern formed nearby the main pattern and not transferred to thetransfer-target surface; characterized in that the main pattern and theassist pattern are each constituted from a semi-transparent film made ofthe same material; and a retardation of 180° is generated between alight transmitting through the main pattern and a light transmittingthrough a transparent region of the transparent substrate, and apredetermined retardation within a scope of 70° to 115° is generatedbetween a light transmitting through the assist pattern and a lighttransmitting through a transparent region of the transparent substrate.

In the above-mentioned invention, preferably, a film thickness of theassist pattern is thinner than a film thickness of the main pattern, anda film thickness difference is a predetermined film thickness differencewithin a scope of 24 nm to 40 nm.

In the above-mentioned invention, the film thickness difference ispreferably formed by dry-etching.

In the above-mentioned invention, an exposing light transmittance of theassist pattern is preferably a predetermined transmittance within ascope of 15% to 29%.

In the above-mentioned invention, the semi-transparent film made of thesame material is preferably a single-layer semi-transparent film or atwo-layer semi-transparent film.

In the above-mentioned invention, preferably, the single-layersemi-transparent film is a semi-transparent film made of a molybdenumsilicide-based material, and the two-layer semi-transparent film is suchthat a semi-transparent film made of a chromium-based material and asemi-transparent film made of a molybdenum silicide-based material aresequentially provided on the transparent substrate.

In the above-mentioned invention, a light shielding region is preferablyformed in an outer periphery of the photomask.

In the above-mentioned present invention, preferably, theabove-mentioned single-layer semi-transparent film is a semi-transparentfilm made of a molybdenum silicide-based material and theabove-mentioned two-layer semi-transparent film is such that asemi-transparent film made of a chromium-based material and asemi-transparent film made of a molybdenum silicide-based material aresequentially provided on the above-mentioned transparent substrate.

In the above-mentioned present invention, preferably, both the mainpattern and the assist pattern are line patterns, and the main patternis an isolated pattern or a periodic pattern.

Furthermore, the present invention provides a manufacturing method of aphotomask using an ArF excimer laser as an exposing source, being usedfor a projection exposure by an off axis illumination, and comprising ona principal plane of a transparent substrate a main pattern transferredto a transfer-target surface by the projection exposure and an assistpattern formed nearby the main pattern and not transferred to thetransfer-target surface; characterized by comprising steps of:

(a) forming a semi-transparent film and a light shielding filmsequentially on the principal plane of the transparent substrate toobtain a film thickness for allowing a retardation of approximately 180°between a light transmitting through the semi-transparent film and alight transmitting through a transparent region of the transparentsubstrate;

(b) forming a first resist pattern on the light shielding film to form amain pattern part and an assist pattern part by sequentially dry-etchingthe light shielding film and the semi-transparent film;

(c) peeling off the first resist pattern to subsequently form a secondresist pattern on the light shielding film and removing the lightshielding film from the assist pattern part by etching;

(d) peeling off the second resist pattern to subsequently dry-etch thewhole principal plane of the transparent substrate and forming theassist pattern by dry-etching the semi-transparent film of the assistpattern part until a film thickness for allowing a predeterminedretardation within a scope of 70° to 115° is obtained between a lighttransmitting through the assist pattern and the light transmittingthrough the transparent region of the transparent substrate; and

(e) removing the light shielding film of the main pattern part byetching to form the main pattern and generate a retardation of 180°between a light transmitting through the main pattern and the lighttransmitting through the transparent region of the transparentsubstrate.

In the above-mentioned present invention, the dry-etching of thesemi-transparent film in the step (d) is half-etching up to a halfwaystage of a film thickness of the semi-transparent film.

Furthermore, the present invention provides a manufacturing method of aphotomask using an ArF excimer laser as an exposing source, being usedfor a projection exposure by an off axis illumination, and comprising ona principal plane of a transparent substrate a main pattern transferredto a transfer-target surface by the projection exposure and an assistpattern formed nearby the main pattern and not transferred to thetransfer-target surface; characterized by comprising steps of:

(a) forming a semi-transparent film and a light shielding filmsequentially on the principal plane of the transparent substrate, inwhich the semi-transparent film is a two-layer semi-transparent film anda lower-layer semi-transparent film on the transparent substrate sideserves also as an etch stop layer of an upper-layer semi-transparentfilm, to obtain a film thickness for allowing a retardation ofapproximately 180° between a light transmitting through the two-layersemi-transparent film and a light transmitting through a transparentregion of the transparent substrate;

(b) forming a first resist pattern on the light shielding film to form amain pattern part and an assist pattern part by sequentially dry-etchingthe light shielding film and the two-layer semi-transparent film;

(c) peeling off the first resist pattern to subsequently form a secondresist pattern on the light shielding film and removing the lightshielding film form the assist pattern part by etching;

(d) peeling off the second resist pattern to subsequently dry-etch thewhole principal plane of the transparent substrate and forming theassist pattern by dry-etching the semi-transparent film of the assistpattern part until a film thickness for allowing a predeterminedretardation within a scope of 70° to 115° is obtained between a lighttransmitting through the assist pattern and the light transmittingthrough the transparent region of the transparent substrate; and

(e) removing the light shielding film of the main pattern part byetching to form the main pattern and generate a retardation of 180°between a light transmitting through the main pattern and the lighttransmitting through the transparent region of the transparentsubstrate.

In the above-mentioned present invention, a film thickness differencebetween a first thickness of the main pattern and a second thickness ofthe assist pattern is preferably a predetermined film thicknessdifference within a scope of 24 nm to 40 nm.

In the above-mentioned present invention, the manufacturing methodpreferably comprises a step of forming a resist pattern for a lightshielding region to form a light shielding region in an outer peripheryof the photomask after the step (d) of forming the assist pattern, andthe light shielding region is formed at the same time of removing thelight shielding film from the main pattern by dry-etching to form a mainpattern.

Furthermore, the present invention provides a photomask comprising on aprincipal plane of a transparent substrate a main pattern transferred toa transfer-target surface by a projection exposure and an assist patternformed nearby the main pattern and not transferred to thetransfer-target surface; characterized in that the main pattern and theassist pattern are each constituted from a semi-transparent film made ofthe same material; and a film thickness of the assist pattern is thinnerthan a film thickness of the main pattern, and a film thicknessdifference is a predetermined film thickness difference within a scopeof 24 nm to 40 nm.

Furthermore, the present invention provides a correcting method of aphotomask using an ArF excimer laser as an exposing source, being usedfor a projection exposure by an off axis illumination, and comprising ona principal plane of a transparent substrate a main pattern transferredto a transfer-target surface by the projection exposure and an assistpattern formed nearby the main pattern, in the case where the assistpattern is resolved on the transfer-target surface by the projectionexposure; characterized in that a surface of the assist pattern to beresolved is etched or ground to thin a film thickness of the assistpattern to be resolved until the assist pattern is not resolved on thetransfer-target surface.

In the above-mentioned present invention, a film thickness differencebetween a film thickness of the assist pattern after being corrected byetching or grinding to thin and a film thickness of the assist patternbefore being corrected is preferably within a scope of 1 nm to 40 nm.

In the above-mentioned present invention, preferably, the etching is agas-assisted etching by using an electron beam of an electron beam maskcorrecting device, and the grinding is a grinding by using a probe of anatomic force microscope.

In the above-mentioned present invention, preferably, the main patternand the assist pattern are each constituted from a semi-transparentfilm, and a film thickness of the main pattern is film thickness forgenerating a retardation of 180° between a light transmitting throughthe main pattern and a light transmitting through a transparent regionof the transparent substrate.

In the above-mentioned present invention, preferably, the main patternis constituted from a light shielding film, and the assist pattern isconstituted from a semi-transparent film.

In the above-mentioned present invention, that the main pattern and theassist pattern are preferably each constituted from a light shieldingfilm.

In the above-mentioned present invention, preferably, both the mainpattern and the assist pattern are line patterns, and the main patternis an isolated pattern or a periodic pattern.

In the above-mentioned present invention, preferably, the assist patternis corrected by the correcting method of a photomask, and the filmthickness of the assist pattern after being corrected is thinner thanthe film thickness of the assist pattern before being corrected.

Advantageous Effects of Invention

According to a photomask of the present invention, in a halftone maskhaving an assist pattern, the formation of only the assist pattern partinto a thin film allows a transferred image having high contrast to beformed while keeping the focal depth magnification effect as the assistpattern. Even though the assist pattern dimension is enlarged from 56 nmto 104 nm, the assist pattern part is not resolved and the focal depthmagnification effect of a main pattern at a repetition end is notinfluenced badly and the assist pattern dimension may be enlarged toapproximately twice the conventional dimension; the decreasing of anaspect ratio of the assist pattern produces the effect of restrainingchip and falling of the assist pattern. Also, with regard to a photomaskof the present invention, mask blanks for a halftone mask conventionallyused may be directly used therefor in the case where a semi-transparentfilm is a single layer; the mask blanks material does not need to bemodified, so that compatibility of the mask blanks may be secured incontrast to a halftone mask using no assist pattern, and qualitymaintenance of the mask and reduction of mask costs become feasible.

According to a manufacturing method of a photomask of the presentinvention, a main pattern and an assist pattern are each constitutedfrom a semi-transparent film made of the same material, so that afilm-forming process of the semi-transparent film is easy; mask blanksfor a halftone mask conventionally used may be directly used therefor inthe case where the semi-transparent film is a single layer, and the maskblanks material does not need to be modified, so that mask productioncosts may be reduced. A space between the main pattern and the assistpattern is widened so more by making the assist pattern width smallerthan the main pattern as to allow a manufacturing method having a highmargin of the misalignment of a first pattern formed on a transparentsubstrate with a second pattern formed subsequently, and allow aphotomask for improving transfer properties of the pattern withoutincreasing difficulty in mask production.

According to a photomask of the present invention, the determination offilm thickness difference between a main pattern and an assist patternat a predetermined scope allows phenomena to be restrained fromoccurring such that part of the assist pattern is chipped, the assistpattern is peeled off from a substrate surface, and the assist patternfalls in the line width direction.

According to a correcting method of a photomask of the presentinvention, in a correcting method of a photomask in the case where anassist pattern which may not originally be transferred to atransfer-target surface is resolved on a transfer-target surface, asurface of the assist pattern to be resolved is etched or ground to thinthe film thickness of the assist pattern until the assist pattern is notresolved on a transfer-target surface, so that a problem such that theassist pattern is transferred may be solved to correct to a photomaskfor forming a transferred image having high contrast while keeping thefocal depth magnification effect as the assist pattern. The correctingmethod of a photomask of the present invention differs from a correctionof the assist pattern in the line width direction as a conventionalmethod, is a correcting method of forming the assist pattern in thethickness direction into a thin film, and allows the photomask havingthe assist pattern to be securely corrected by an easy method.

According to a photomask by a correcting method of the presentinvention, with regard to the photomask in which an assist pattern istransferred to a transfer-target surface, the assist pattern iscorrected in the thickness direction, so that the assist pattern is nottransferred to a transfer-target surface and the effect of allowing ahigh-quality photomask for forming a transferred image having highcontrast while having the focal depth magnification effect is produced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a partial cross-sectional schematic view showing an embodimentof a halftone mask having an assist pattern of the present invention.

FIG. 2 is a partial cross-sectional schematic view showing anotherembodiment of a halftone mask having an assist pattern of the presentinvention.

FIGS. 3A and 3B show a Cquad pupil filter used for evaluating a halftonemask (or a photomask having an assist pattern) of the present invention;FIG. 3A is a plan schematic view of Cquad, and FIG. 3B is a perspectiveschematic view in irradiating the mask with exposing light by usingCquad.

FIGS. 4A and 4B are an evaluation pattern used in a halftone mask (or aphotomask having an assist pattern) of the present invention, and a viewof an aerial image showing a relation between a position of theevaluation pattern and light intensity.

FIG. 5 is a view showing a relation between SRAF film thicknessdifference and SRAF light intensity/slice level in changing CD of SRAF.

FIG. 6 is a view showing a relation between line CD and Defocus at amain pattern end on a wafer in changing CD of SRAF.

FIGS. 7A to 7E are a process cross-sectional schematic view showing afirst embodiment of a manufacturing method of a photomask of the presentinvention.

FIGS. 8A to 8E are a process cross-sectional schematic view showing asecond embodiment of a manufacturing method of a photomask of thepresent invention.

FIGS. 9A to 9F are a process cross-sectional schematic view showing athird embodiment of a manufacturing method of a photomask of the presentinvention.

FIGS. 10A to 10D are a process cross-sectional schematic view showing afourth embodiment of a manufacturing method of a photomask of thepresent invention.

FIGS. 11A to 11E are a process cross-sectional schematic view showing anembodiment of a manufacturing method of a conventional photomask.

FIG. 12 is a view showing a relation between SRAF etching amount (on amask) and SRAF CD (a dimension on a wafer) in the embodiment shown inFIG. 3.

FIG. 13 is a view showing an influence of SRAF etching amount error onmain pattern CD in the embodiment shown in FIGS. 3A and 3B.

FIG. 14 is a view showing a relation between main pattern CD at arepetition end on a wafer and Defocus in changing SRAF etching amount inthe embodiment shown in FIGS. 3A and 3B.

FIG. 15 is a view showing light intensity distribution of a main patternat a repetition end on a wafer in changing SRAF etching amount in theembodiment shown in FIGS. 3A and 3B.

FIG. 16A is a plan schematic view of a Quasar pupil filter used forsimulation, FIG. 16B is a perspective schematic view in irradiating amask with exposing light by using Quasar, and FIG. 16C is a planschematic view of a mask pattern 164.

FIG. 17 is a view showing a relation between SRAF etching amount (on amask) and SRAF CD (a dimension on a wafer) in the embodiment shown inFIGS. 16A to 16C.

FIG. 18 is a view showing an influence of SRAF etching amount error on amask on main pattern CD error on a wafer in the embodiment shown inFIGS. 16 A to 16C.

FIG. 19 is a view showing a relation between main pattern CD and Defocusin changing SRAF etching amount in the embodiment shown in FIGS. 16 A to16C.

FIG. 20 shows a relation between mask CD and NILS in conventionalhalftone mask and binary mask.

FIG. 21 shows a relation between mask CD and MEEF in conventionalhalftone mask and binary mask.

FIG. 22 is a view showing mask CD and exposure latitude in conventionalhalftone mask and binary mask.

FIG. 23 is a view showing a ratio of SRAF part light intensity to slicelevel of light intensity threshold with respect to SRAF CD on a wafer inconventional halftone mask and binary mask.

FIGS. 24A and 24B are a plan view and a longitudinal cross-sectionalview of a photomask having a conventional semi-transparent assistpattern described in Patent Literature 2.

FIGS. 25A to 25C area cross-sectional schematic view showing anembodiment of a correcting method of a photomask having an assistpattern of the present invention.

FIGS. 26A to 26F are a partial cross-sectional schematic view showing anexample of a photomask having an assist pattern to which a correctingmethod of the present invention is applicable.

FIG. 27 is an SEM photograph of an SRAF part after being formed into athin film by a test sample in Examples.

FIG. 28 is an image showing a planar state of light intensitydistribution on a wafer by a lithographic simulation microscope afterpartially etching an SRAF part in a test sample.

FIG. 29 is a view of an aerial image showing a relation between apattern position and light intensity of an SRAF part before being formedinto a thin film in Examples.

FIG. 30 is a partial magnified view of FIG. 29 and is a view of anaerial image showing a relation between a pattern position and lightintensity of a main pattern at a repetition end and an assist pattern S1before forming SRAF into a thin film.

FIG. 31 is a comparison reference view with respect to FIG. 30 and is aview of an aerial image showing a relation between a pattern positionand light intensity of a main pattern at a repetition end and an assistpattern S1 in the case of no SRAF.

FIG. 32 is a view of an aerial image showing a relation between apattern position and light intensity of an SRAF part after being formedinto a thin film in Examples.

FIG. 33 shows a situation of FIGS. 27 and 28 inside a broken line, andis a partial magnified view of FIG. 32 while is a view of an aerialimage showing a relation between a pattern position and light intensityof a main pattern at a repetition end and an assist pattern S1 afterforming SRAF into a thin film.

FIG. 34 is a view showing focal depth of a main pattern at a repetitionend by forming SRAF into a thin film.

DESCRIPTION OF EMBODIMENTS A. Photomask

A photomask of the present invention is a mask which uses an ArF excimerlaser as an exposing source, is used for a projection exposure by an offaxis illumination, and is preferably intended to be used for forming aminute semiconductor device with a half pitch of 65 nm or less, further45 nm and 32 nm on a wafer.

(Transfer Properties of Conventional Halftone Mask)

The transfer properties of a halftone mask having an assist pattern as atarget of the present invention are first described before describingthe present invention. The inventors of the present invention haveexamined the transfer properties of a halftone mask having an assistpattern for forming a minute pattern with a half pitch of 45 nm or lesson a wafer by simulation while comparing with a binary mask by using aconventional halftone mask.

Conventionally, the evaluation of the transfer properties of a maskpattern has been predicted by a method of mainly expressing the planarproperties of the mask pattern with transmittance and retardation. Inrecent years, indices such as contrast or NILS (Normalized ImageLog-Slope) and MEEF (Mask Error Enhancement Factor) have been used forevaluating the transfer properties of a photomask. First, the transferproperties of the mask were evaluated by using NILS and MEEF.

NILS is represented by the following expression (1). A larger value ofNILS brings a steeper optical image and improved dimensioncontrollability of a resist pattern. Generally, NILS is preferably 2 ormore, and a resist process to be resolved even at an NILS ofapproximately 1.5 or more has been requested in accordance withmicronization of a semiconductor device. Here, W is a desired patterndimension, I_(th) is a light intensity threshold for allowing W, and(dI/dx) is a slope of an aerial image.NILS=(dI/dx)/(W×I _(th))  (1)

MEEF is represented by the following expression (2) and shown by a ratioof a pattern dimension change amount on a wafer (Δ wafer CD) to a maskdimension change amount (Δ mask CD). CD shows an important dimension(Critical Dimension) of a mask and a wafer. A numerical value 4 of theexpression (2) is a contraction ratio of a mask and exemplifies the caseof using a general quadruple mask. As shown by the expression (2), asmaller value (nearby 1) of MEEF allows a mask pattern to be transferredto a wafer pattern more accurately; a smaller value of MEEF improves awafer production yield and consequently also a mask production yieldused for the wafer production.MEEF=Δwafer CD/Δmask CD/4  (2)

In the present invention, EM-Suite (trade name, manufactured byPanoramic Technology Inc.) was used as simulation software forestimating the transfer properties of a mask pattern. The mainsimulation conditions were such that an ArF excimer laser (193 nm) wasused as an illuminating source, NA was 1.35, and a Cquad pupil filter 31shown in FIGS. 3A and 3B was used as an off axis illumination. FIG. 3Ais a plan schematic view of the Cquad 31, and FIG. 3B is a perspectiveschematic view in irradiating a mask 33 with exposing light by using theCquad 31. The Cquad 31 was such that an angular aperture of a fan-shapedlight transmission part was 35°, an outside diameter was 0.9, and aninside diameter was 0.7 (a radius of the pupil filter is regarded as 1).A conventional general molybdenum silicide-based halftone mask with atransmittance of 6% in an exposure wavelength of 193 nm (referred to as6%-halftone) and a molybdenum silicide-based binary mask for comparisonwere used as the mask 33. A target line dimension on a wafer was 45 nm,and the pattern was a line/space repetition pattern with a pitch of 90nm (a half pitch of 45 nm).

FIGS. 20 and 21 are views showing a relation between a mask bias andtransfer properties in a transfer-target dimension of 45 nm on a waferobtained by the above-mentioned simulation in conventional halftone maskand binary mask; FIGS. 20 and 21 show a relation of mask CD to NILS andMEEF, respectively,

In NILS shown in FIG. 20, with regard to the halftone mask, NILS showsthe maximum value in a mask CD of 32 nm to 44 nm (on a wafer), in whicha line pattern dimension is thinned by determining a mask bias at theminus side. On the other hand, with regard to the binary mask, the morea line pattern dimension is thickened by determining a mask bias at theplus side, the more NILS tends to increase.

In MEEF shown in FIG. 21, with regard to both the halftone mask and thebinary mask, the more a line pattern dimension is thinned by determininga mask bias at the minus side, the more MEEF decreases; it is morepreferable that the halftone mask offers a smaller value than the binarymask.

Through FIGS. 20 and 21, with regard to the halftone mask, the mask CDof the maximum NILS and the minimum MEEF corresponds approximately. Onthe other hand, with regard to the binary mask, NILS and MEEF are in areciprocal relation and it is found that when one property is improved,the other property is deteriorated. This fact shows that the halftonemask is more appropriate than the binary mask for forming a pattern witha half pitch of 45 nm or less. Accordingly, as described in the presentinvention, it is one of preferable choices to use the halftone mask as aphotomask for forming a pattern with a half pitch of 45 nm or less.

FIG. 22 is a view showing mask CD and Exposure Latitude (also referredto as exposure tolerance) of conventional halftone mask and binary mask.The exposure latitude is a value showing exposure margin for obtainingfavorable resist dimension and shape. Here, the exposure latitude wasevaluated on the conditions that the error of wafer transfer CD became±3.8 nm or less in the case where a focal plane was shifted within thescope of ±50 nm and main pattern mask CD was shifted within the scope of±2.5 nm. Here, mask CD is converted onto a wafer and mask CD of the mainpattern to be transferred is shown. In FIG. 22, the halftone mask (adotted line in the FIG.) offers the best exposure latitude of 8.3% whenmask CD is 32 nm, and offers the same exposure latitude as the binarymask (a solid line) when mask CD is 40 nm. On the other hand, the binarymask offers the best exposure latitude of 7% when mask CD is 46 nm;however, the exposure latitude is smaller as compared with the halftonemask.

FIGS. 4A and 4B show an evaluation pattern used in the present invention(FIG. 4A), and a view of an aerial image showing light intensitycorresponding to a position of the evaluation pattern (FIG. 4B). Theevaluation pattern is regarded as a pair including nine line/space witha half pitch of 45 nm as the main pattern and two SRAF (the half pitchof SRAF is the same as the main pattern) at both ends of the mainpattern for improving resolution of the main pattern at the ends, and isa repetition pattern holding a space of 400 nm therebetween. Both themain pattern and SRAF are the above-mentioned 6%-halftone.

Next, in a halftone mask having an assist pattern, transfer propertiesof the assist pattern (SRAF) at the ends of the line/space pattern aredescribed. In FIGS. 4A and 4B, the horizontal axis shows a position ofthe main pattern and a pair of the pattern of SRAF, and the verticalaxis shows normalized light intensity when light intensity of atransmission part with no patterns is regarded as 1; slice level shownby a horizontal solid line in the drawing is normalized light intensitythreshold. The slice level varies with the dimension of the main maskpattern. The case where the minimum light intensity of an SRAF partshown by an arrow in the drawing becomes lower than the slice levelsignifies that SRAF is resolved on a wafer.

FIG. 23 is a view showing a ratio (the vertical axis) of SRAF part lightintensity to slice level of normalized light intensity threshold withrespect to SRAF CD (the horizontal axis) on a wafer in a halftone maskand a binary mask in the case of making the film thickness constant onthe basis of prior art. The halftone mask (a triangular point in theFIG.) shows the case where the main pattern CD is of three kinds (32 nm;36 nm; and 40 nm on a wafer). The above-mentioned ratio of 1 or lesscauses SRAF to be transferred, so that the above-mentioned ratio needsto be 1 or more for not transferring SRAF. When the main pattern CD ofthe halftone mask shown by a dotted line in the drawing is 32 nm (128 nmon the mask), the above-mentioned exposure latitude exhibits the bestvalue; however, if SRAF CD is not 14 nm (56 nm on the mask) or less,SRAF is resolved to find the mask production difficult.

The above is a simulation result in the case of using a conventional6%-halftone mask having SRAF, and is found excellent in mask propertiesin the simulation; yet, the SRAF dimension becomes so extremely smallthat the actual mask production is difficult.

(Photomask of the Present Invention)

Next, embodiments of a photomask and a manufacturing method of thephotomask of the present invention are described in detail on the basisof drawings while referring to the above-mentioned results. In thepresent invention, except for the case where SRAF is between theafter-mentioned main patterns, in describing transfer properties of thefollowing mask pattern, a Cquad pupil filter 31 shown in theabove-mentioned FIGS. 3A and 3B was used and EM-Suite (trade name,manufactured by Panoramic Technology Inc.) was used as simulationsoftware. The main simulation conditions are such that an ArF excimerlaser (193 nm) is used as an illuminating source and NA is 1.35. Thepattern shown in the above-mentioned FIG. 4A is used for an evaluationpattern.

First Embodiment

FIG. 1 is a partial cross-sectional schematic view showing a firstembodiment of a halftone mask having an assist pattern as a photomask ofthe present invention, exemplifies the case where a line/space patternis provided. FIG. 1 is a halftone mask 10 in which a main pattern 12 isprovided on a transparent substrate 11 such as a synthetic quartzsubstrate by a single-layer semi-transparent film for transmittingexposing light at a predetermined transmittance to change a phase, andan assist pattern (SRAF) 13 composed of a single-layer semi-transparentfilm made of the same material as the main pattern 12 is formed nearbythe main pattern 12. FIG. 1 exemplifies only two lines of the mainpattern 12 and the assist pattern 13 each and part of a mask pattern,and needless to say, is not limited thereto. The main pattern may be anisolated pattern or a periodic pattern.

The halftone mask 10 having an assist pattern of the present inventionis determined so that a retardation of 180° is generated between thelight transmitting through the main pattern 12 and the lighttransmitting through a transparent region with no patterns of thetransparent substrate 11, and a predetermined retardation within thescope of 70° to 115° is generated between the light transmitting throughthe assist pattern 13 and the light transmitting through a transparentregion of the transparent substrate 11. The determination ofretardations of the main pattern 12 and the assist pattern 13 asdescribed above allows the halftone mask 10 not to resolve the assistpattern 13 while keeping the focal depth magnification effect as theassist pattern, and to form a transferred image having high contrast ofthe main pattern 12.

In order to generate the above-mentioned retardations, with regard tothe halftone mask 10 having the assist pattern of the present invention,the film thickness of the assist pattern 13 is thinner than the filmthickness of the main pattern 12, and the film thickness difference(hereinafter referred to as SRAF film thickness difference) isdetermined at a predetermined film thickness difference within the scopeof 24 nm to 40 nm. The above-mentioned predetermined film thicknessdifference may be formed by selectively dry-etching an SRAF part.

With regard to the halftone mask 10 having the assist pattern, forexample, when ArF exposing light transmittance of the main pattern forgenerating a retardation of 180° is 6%, ArF exposing light transmittanceof the assist pattern for generating the above-mentioned predeterminedretardation within the scope of 70° to 115° is a predeterminedtransmittance within the scope of 15% to 29%.

A material for a semi-transparent film composing the main pattern 12 andthe assist pattern 13 of the halftone mask 10 of the present inventionshown in FIG. 1 is not particularly limited and examples thereof includesemi-transparent films such as a molybdenum silicide oxide film (MoSiO),a molybdenum silicide nitride film (MoSiN) and a molybdenum silicideoxide nitride film (MoSiON), which are molybdenum silicide-basedmaterials. The molybdenum silicide-based semi-transparent films are putto practical use as a halftone mask material and are more preferablematerials.

A conventionally known method may be applied to the formation of thesemi-transparent film 12; for example, a molybdenum silicide oxide film(MoSiO) may be formed into a thickness of several tens of nm by areactive sputtering method in a mixed gas atmosphere of argon and oxygenwith the use of a mixed target (Mo:Si=1:2% by mol) of molybdenum andsilicon.

In the case where the semi-transparent film composing the main pattern12 and the assist pattern 13 is a semi-transparent film made ofmolybdenum silicide-based materials, a pattern may be formed bydry-etching with the use of fluorine-based gas such as CF₄, CHF₃ andC₂F₆, or mixed gas thereof, or alternatively a gas, with which oxygen ismixed, as etching gas.

Here, in the case where the semi-transparent film is a single layer ofmolybdenum silicide-based materials, ordinarily the transparentsubstrate surface also is slightly etched and dug in on the occasion ofdry-etching the semi-transparent film to form a mask pattern (not shownin FIG. 1). In the present invention, the digging depth of thetransparent substrate surface of a part with no mask patterns ispreferably controlled to a depth within the scope of 0 to 10 nm. Adigging depth of more than 10 nm brings no good influence on maskproperties. Thus, in a halftone mask of the present invention, theetching depth of the transparent substrate surface is controlled to apredetermined depth within the scope of 0 to 10 nm to previouslydetermine retardation including this depth. In the followingembodiments, the digging depth of any halftone mask to be etched isdetermined at 4 nm, and it is needless to say that another etching depthmay be used if it is within the scope of 0 to 10 nm.

For example, in the case where molybdenum silicide with a film thicknessof 68 nm is used as the semi-transparent film, a halftone mask of theembodiments may be such that the main pattern (a film thickness of 68nm) has ArF excimer laser light transmittance of 6% and a retardation of180° with a transparent region of the transparent substrate, and theassist pattern has a predetermined film thickness difference within thescope of 24 nm to 40 nm with the main pattern and a predeterminedretardation within the scope of 70° to 115° with a transparent region ofthe transparent substrate.

Second Embodiment

In order to decrease the above-mentioned digging of the transparentsubstrate surface, a halftone mask composed of a two-layersemi-transparent film shown in FIG. 2 is shown as another embodiment ofa photomask of the present invention. A main pattern and an assistpattern are composed of a two-layer semi-transparent film made of thesame material, and a lower-layer semi-transparent film 24 on thetransparent substrate side has the function of an etch stop layer duringdry-etching of an upper-layer semi-transparent film 25 and also has thefunction as a semi-transparent film. Examples of the upper-layersemi-transparent film 25 include the above-mentioned molybdenumsilicide-based materials. In this case, the lower-layer semi-transparentfilm 24 is preferably a chromium oxide film (CrO), a chromium nitridefilm (CrN) and a chromium oxide nitride film (CrON), which arechromium-based materials. The reason therefor is that a thin film madeof the above-mentioned chromium-based materials is semi-transparentagainst exposing light and resistant to fluorine-based gas used fordry-etching molybdenum silicide-based materials. The chromium-basedmaterials are formed by a conventionally known reactive sputteringmethod, and a thin film made of the chromium-based materials in anunwanted part may be dry-etched by chlorine-based gas and does notdamage the transparent substrate. The upper-layer semi-transparent film25 and the lower-layer semi-transparent film 24 are formed into filmswith a thickness of several tens nm and several nm to several tens nm,respectively.

With regard to a halftone mask of the present invention, in theabove-mentioned first and second embodiments, a light shielding regionmay be formed in the outer periphery of the mask. Ordinarily, the maskouter periphery is subject to multiple exposure in a projection exposureon a semiconductor wafer, so that a photomask provided with a lightshielding region in the mask outer periphery is used. Accordingly, alsoin the present invention, a light shielding film is provided on asemi-transparent film in a desired region such as the outer periphery toallow a light shielding region. The light shielding film is formed as alight shielding region in such a manner that a metal film such aschromium having light shielding property is formed into a thickness ofapproximately several tens nm to 200 nm and patterned.

(Transfer Properties of Assist Pattern)

Next, the effect of thin-film forming for an assist pattern (SRAF) of ahalftone mask of the present invention shown in FIG. 1 is described.FIG. 5 is a view showing a relation between SRAF film thicknessdifference (the horizontal axis) and SRAF light intensity/slice level ofnormalized light intensity threshold (the vertical axis) in changing CDof SRAF in a halftone mask such that CD of a main pattern on a wafer is32 nm. It is shown that SRAF is resolved on a wafer unless SRAF lightintensity/slice level is 1 or more.

As shown in FIG. 5, when CD of SRAF is as minute as 14 nm (56 nm on amask), SRAF is not transferred even though SRAF film thicknessdifference is 0, namely, the same as the film thickness (68 nm) of themain pattern. When CD of SRAF is 22 nm (88 nm on a mask), SRAF is notresolved and transferred if SRAF film thickness difference is 24 nm ormore. Similarly, when CD of SRAF is 26 nm (104 nm on a mask), SRAF isnot transferred if SRAF film thickness difference is 30 nm or more, andwhen CD of SRAF is 30 nm (120 nm on a mask), SRAF is not transferred ifSRAF film thickness difference is 34 nm or more.

As described in the above-mentioned FIG. 23, in a halftone mask suchthat conventional main pattern and assist pattern (SRAF) are composed ofthe same material and the same film thickness, when CD of the mainpattern is determined at 32 nm, SRAF is used only if CD of SRAF is 14 nmor less; however, as described above, the use of SRAF of the presentinvention, which is formed into a thin film, allows SRAF to be usedwithout being resolved and transferred even though CD of SRAF isenlarged to approximately twice the conventional dimension, that is, 26nm to 30 nm. SRAF may be easily formed into a thin film by selectivelydry-etching an SRAF part. The SRAF dimension may be enlarged toapproximately twice the conventional dimension, so that a halftone maskhaving SRAF made of the same material, which is conventionally difficultto be micronized and used, may be used.

FIG. 6 is a view showing a relation between line CD and Defocus (focalposition change) at a main pattern end on a wafer in changing CD ofSRAF. A predetermined film thickness difference (24 nm, 32 nm and 40 nm)with film thickness of the main pattern is offered by etching so thatSRAF is not resolved with respect to each CD of SRAF. As shown in FIG.6, CD of SRAF is enlarged to 22 nm to 30 nm to thin the film thicknessof SRAF, so that approximately the same tendency is exhibited withoutany CD change among each of SRAF dimensions when the focus is changed.That is to say, the formation of SRAF into a thin film has no badinfluence on Defocus to allow the same dimensional accuracy.

As described above, with regard to a photomask of the present invention,the formation of only the assist pattern part into a thin film allows atransferred image having high contrast to be formed while keeping thefocal depth magnification effect as the assist pattern. In addition, theassist pattern dimension may be enlarged to approximately twice theconventional dimension; the decreasing of an aspect ratio of the assistpattern allows the effect of reducing chip and falling of the assistpattern. Also, in the case where a molybdenum silicide-basedsingle-layer film is used as a photomask of the present invention, maskblanks for a halftone mask conventionally used may be directly used; themask quality is maintained to allow a mask having a high-precisionminute pattern to be used.

B. Manufacturing Method of Photomask

Next, a manufacturing method of a photomask of the present invention isdescribed. As described above, the photomask of the present invention ischaracterized in that a predetermined retardation within the scope of70° to 115° is generated between the light transmitting through theassist pattern and the light transmitting through the transparent regionof the transparent substrate; in order to generate the above-mentionedretardation in the assist pattern, the film thickness of the assistpattern is so thinner than the film thickness of the main pattern as toallow a predetermined film thickness difference within the scope of 24nm to 40 nm. Examples of a method of allowing a predetermined filmthickness difference include a method of changing film thickness to beformed in accordance with a pattern during the formation of asemi-transparent film, and a method of changing film thickness byetching a semi-transparent film in accordance with a pattern after theformation of the semi-transparent film. The manufacturing method of aphotomask of the present invention is based on the latter etching methodfor offering easy production to allow a high-precision mask.

(Conventional Manufacturing Method of Photomask)

The problems in the case of producing a photomask of the presentinvention by using a publicly known general manufacturing method aredescribed before describing a manufacturing method of a photomask of thepresent invention, and subsequently a manufacturing method of aphotomask of the present invention is described.

FIGS. 11A to 11E are a process cross-sectional schematic view in thecase of producing a photomask of the present invention by using apublicly known conventional manufacturing method. As shown in FIGS. 11Ato 11E, a semi-transparent film 112 is formed on a transparent substrate111 to obtain film thickness for allowing a retardation of 180° betweenthe light transmitting through the semi-transparent film and the lighttransmitting through a transparent region of the transparent substrate,and subsequently form a light shielding film 113 on the semi-transparentfilm (FIG. 11A). Next, a first resist pattern 114 is formed on the lightshielding film 113, and the light shielding film 113 and thesemi-transparent film 112 are sequentially dry-etched to form a mainpattern part 115 and an assist pattern part 116 (FIG. 11B). Next, thefirst resist pattern 114 is peeled off to remove the exposed lightshielding film of the pattern parts by etching (FIG. 11C). Subsequently,the main pattern part 115 is covered with a second resist pattern 117 toform an assist pattern 118 by dry-etching the semi-transparent film ofthe assist pattern part until film thickness for allowing apredetermined retardation between the light transmitting through theassist pattern part and the light transmitting through the transparentregion of the transparent substrate is obtained (FIG. 11D), and obtain ahalftone mask 110 by peeling off the second resist pattern 117 (FIG.11E).

However, in the above-mentioned manufacturing method, the transparentsubstrate 111 surface not covered with the second resist pattern 117 issimultaneously etched during dry-etching of the semi-transparent film ofthe assist pattern part 116; as shown in (FIG. 11E), a level difference121 is caused on the transparent substrate 111 surface on a boundarysurface of the resist pattern 117 to cause a problem such that the maskquality is deteriorated and not put to practical use. Accordingly, theconventional mask manufacturing method as described above may not beapplied to the production of a photomask of the present invention.

(Manufacturing Method of Photomask of the Present Invention)

First Embodiment

Thus, a manufacturing method of a photomask of the present invention isa manufacturing method for solving the above-mentioned problems, and amanufacturing method of a photomask which uses an ArF excimer laser asan exposing source, is used for a projection exposure by an off axisillumination, and is provided on a transparent substrate with a mainpattern to be transferred to a transfer-target surface by a projectionexposure and an assist pattern to be formed nearby the main pattern andnot transferred to a transfer-target surface.

FIGS. 7A to 7E is a process cross-sectional schematic view showing afirst embodiment of a manufacturing method of a photomask of the presentinvention shown in FIG. 1. As shown in FIG. 7A, a semi-transparent film72 is formed on a transparent substrate 71 such as a synthetic quartzsubstrate to obtain film thickness for allowing a retardation ofapproximately 180° between the light transmitting through thesemi-transparent film 72 and the light transmitting through atransparent region of the transparent substrate 71, and subsequentlyprepare photomask blanks such that a light shielding film 73 is formedon the above-mentioned semi-transparent film 72.

A conventionally known method may be applied to the formation of thesemi-transparent film 72 and the light shielding film 73; for example, amolybdenum silicide oxide film (MoSiO) as the semi-transparent film 72may be formed by a reactive sputtering method in a mixed gas atmosphereof argon and oxygen with the use of a mixed target (Mo:Si=1:2% by mol)of molybdenum and silicon. Also, in the case where the light shieldingfilm 73 is a metal film such as chromium, a predetermined film thicknessmay be formed by a sputtering method.

The reason why the film thickness of the above-mentionedsemi-transparent film 72 is determined at film thickness for allowing alight retardation of approximately 180° is as follows. In the case wherethe semi-transparent film 72 is dry-etched to form a mask pattern,ordinarily the transparent substrate 71 surface is also etched slightly.The etching depth is preferably 4 nm and the upper limit thereof isdetermined at 10 nm in the present invention. The etching depth of morethan 10 nm brings no good influence on mask properties. Thus, in ahalftone mask of the present invention, the etching depth of thetransparent substrate 71 surface during dry-etching of thesemi-transparent film 72 is controlled to a predetermined depth withinthe scope of 0 to 10 nm to determine retardation while previouslyincluding this depth. Accordingly, the thickness of the semi-transparentfilm in forming is determined at film thickness for allowing aretardation of approximately 180° in previous consideration offluctuation due to etching of the transparent substrate to finally allowa retardation of 180° after forming the main pattern. The followingembodiments are described while determining the above-mentionedpredetermined etching depth at 4 nm as an example. In the presentinvention, an atomic force microscope (AFM) was used for measuring filmthickness and a phase shift amount measuring apparatus (MPM193™:manufactured by Lasertec Corporation) was used for measuringretardation.

Next, a first resist pattern 74 is formed on the above-mentioned lightshielding film 73, and the light shielding film 73 and thesemi-transparent film 72 are sequentially dry-etched into a pattern toform a main pattern part 75 and an assist pattern part 76 (FIG. 7B).

Next, the above-mentioned first resist pattern 74 is peeled off, and asecond resist pattern 77 is formed on the light shielding film to removethe light shielding film 73 in the assist pattern part 76 by etching(FIG. 7C).

In the case where the semi-transparent film 72 is a semi-transparentfilm made of molybdenum silicide-based materials, a pattern may beformed by dry-etching with the use of fluorine-based gas such as CF₄,CHF₃ and C₂F₆, or mixed gas thereof, or alternatively a gas, with whichoxygen is mixed, as etching gas. In the case where the light shieldingfilm 73 is made of chromium, a pattern may be formed by dry-etching withthe use of mixed gas of Cl₂ and oxygen as etching gas without damagingthe semi-transparent film 72 and the transparent substrate 71. In theabove-mentioned process of FIG. 7C, the light shielding film 73 may bealso removed by not dry-etching but wet-etching in an aqueous solutionof a ceric ammonium nitrate salt.

Subsequently, the second resist pattern 77 is peeled off, and the wholeprincipal plane of the transparent substrate 71 is dry-etched on theetching conditions of the semi-transparent film 72 to form an assistpattern 78 by dry-etching the semi-transparent film of the assistpattern part until film thickness for allowing a predeterminedretardation within the scope of 70° to 115° between the lighttransmitting through the assist pattern and the light transmittingthrough a transparent region of the transparent substrate 71 is obtained(FIG. 7D). The etching amount of the assist pattern 78 for allowing theabove-mentioned retardation corresponds to a predetermined filmthickness difference within the scope of 24 nm to 40 nm in filmthickness difference from the semi-transparent film of the main patternpart. The main pattern part is not etched by reason of being coveredwith the light shielding film 73, and film thickness in forming thesemi-transparent film is retained. In the process of FIG. 7D, thedry-etching allows the whole mask to be etched uniformly with highprecision and allows retardation of the assist pattern 78 to becontrolled to a predetermined value with high precision.

Next, the light shielding film of the main pattern part is removed byetching to form a main pattern 79 and then a halftone mask 70 having theassist pattern for generating a retardation of 180° between the lighttransmitting through the main pattern 79 and the light transmittingthrough a transparent region of the transparent substrate 71 (FIG. 7E).In the process of FIG. 7E, the light shielding film 73 may be removed byeither method of dry-etching or wet-etching.

The manufacturing method of a photomask according to the above-mentionedfirst embodiment allows the high-quality halftone mask 70 having theassist pattern 78 without causing the level difference as described inFIGS. 11A to 11E on the transparent substrate 71 surface.

For example, in the case where molybdenum silicide with a film thicknessof 68 nm is used as the semi-transparent film, a high-quality halftonemask such that the main pattern (a film thickness of 68 nm) has ArFexcimer laser light transmittance of 6% and a retardation of 180° with atransparent region of the transparent substrate, and the assist patternhas a predetermined film thickness difference within the scope of 24 nmto 40 nm with the main pattern and a predetermined retardation withinthe scope of 70° to 115° with a transparent region of the transparentsubstrate may be easily produced.

Second Embodiment

FIGS. 8A to 8E are a process cross-sectional schematic view showing asecond embodiment of a manufacturing method of a photomask of thepresent invention shown in FIG. 1; similarly to FIG. 7A, asemi-transparent film 82 is formed on a transparent substrate 81 toobtain film thickness for allowing a retardation of approximately 180°between the light transmitting through the semi-transparent film 82 andthe light transmitting through a transparent region of the transparentsubstrate 81, and subsequently prepare photomask blanks such that alight shielding film 83 is formed on the above-mentionedsemi-transparent film 82 (FIG. 8A).

Next, a first resist pattern 84 is formed on the light shielding film83, and the light shielding film 83 and the semi-transparent film 82 aresequentially dry-etched to stop etching in the halfway stage ofhalf-etching the semi-transparent film 82. In this stage, a thin layerof the semi-transparent film 82 to be removed remains partially on thetransparent substrate 81 in a state of being half-etched, and yet a mainpattern part 85 and an assist pattern part 86 are formed in a state ofhaving the half-etching part (FIG. 8B). The film thickness of thehalf-etching part of the half-etched semi-transparent film 82 in thisstage is previously determined so as to become film thickness to besimultaneously removed by etching during etching of the assist patternin the later process.

Next, the above-mentioned first resist pattern 84 is peeled off, and asecond resist pattern 87 is formed on the light shielding film to removethe light shielding film in the assist pattern part by etching (FIG.8C). In the process of FIG. 8C, the light shielding film 83 may beremoved by either method of dry-etching or wet-etching.

Subsequently, the second resist pattern 87 is peeled off, and the wholeprincipal plane of the transparent substrate 81 is dry-etched on theetching conditions of the semi-transparent film 82 to form an assistpattern 88 by dry-etching the semi-transparent film of the assistpattern part until film thickness for allowing a predeterminedretardation within the scope of 70° to 115° between the lighttransmitting through the assist pattern and the light transmittingthrough a transparent region of the transparent substrate 81 is obtained(FIG. 8D). The etching amount of the assist pattern 88 for allowing theabove-mentioned retardation corresponds to a predetermined filmthickness difference within the scope of 24 nm to 40 nm in filmthickness difference from the main pattern. At this time, thehalf-etching part of the semi-transparent film 82 remaining in a stateof being half-etched is etched simultaneously. The main pattern part isnot etched by reason of being covered with the light shielding film 83.

Next, the light shielding film of the main pattern part is removed byetching to form a main pattern 89 and then a halftone mask 80 having theassist pattern 88 for generating a retardation of 180° between the lighttransmitting through the main pattern 89 and the light transmittingthrough a transparent region of the transparent substrate 81 (FIG. 8E).In the process of FIG. 8E, the light shielding film 83 may be removed byeither method of dry-etching or wet-etching.

The manufacturing method of a photomask according to the above-mentionedsecond embodiment allows the high-quality halftone mask 80 having theassist pattern 88 without causing the level difference as described inFIGS. 11A to 11E on the transparent substrate 81 surface.

Third Embodiment

FIGS. 9A to 9F are a process cross-sectional schematic view showing anembodiment of a manufacturing method of a photomask of the presentinvention shown in FIG. 2. As shown in FIG. 9A, a semi-transparent film92 a and a semi-transparent film 92 are sequentially formed on atransparent substrate 91 such as a synthetic quartz substrate to form atwo-layer semi-transparent film. The lower-layer semi-transparent film92 a has the function of an etch stop layer during dry-etching of theupper-layer semi-transparent film 92 and also has the function as a maskmaterial for a semi-transparent film. The film thickness for allowing aretardation of approximately 180° between the light transmitting throughthe two-layer semi-transparent film and the light transmitting through atransparent region of the transparent substrate 91 is obtained tosubsequently prepare photomask blanks such that a light shielding film93 is formed on the above-mentioned two-layer semi-transparent film.

A conventionally known method may be applied to the formation of thesemi-transparent film 92 a, the semi-transparent film 92 and the lightshielding film 93. For example, a chromium oxide film (CrO), a chromiumnitride film (CrN) and a chromium oxide nitride film (CrON), which arechromium-based materials, are used as the lower-layer semi-transparentfilm 92 a. The reason therefor is that a thin film made of theabove-mentioned chromium-based materials is semi-transparent againstexposing light and resistant to fluorine-based gas used for dry-etchingmolybdenum silicide-based materials. The chromium-based materials may beformed by a conventionally known reactive sputtering method. Examples ofthe upper-layer semi-transparent film 92 include the above-mentionedmolybdenum silicide-based materials. The semi-transparent film 92 of amolybdenum silicide oxide film (MoSiO) may be formed by a reactivesputtering method in a mixed gas atmosphere of argon and oxygen with theuse of a mixed target (Mo:Si=1:2% by mol) of molybdenum and silicon.Chromium is used for the light shielding film 93 and a predeterminedfilm thickness may be formed by a sputtering method.

Next, a first resist pattern 94 a is formed on the above-mentioned lightshielding film 93, and the light shielding film 93, the semi-transparentfilm 92 and the semi-transparent film 92 a are sequentially dry-etchedinto a pattern to form a main pattern part 95 and an assist pattern part96 (FIG. 9B). The transparent substrate 91 is not damaged during etchingof the semi-transparent film 92 a.

In the process of FIG. 9B, in the case where the light shielding film 93is made of chromium, a pattern may be formed by dry-etching with the useof mixed gas of Cl₂ and oxygen as etching gas without damaging thesemi-transparent film and the transparent substrate. In the case wherethe semi-transparent film 92 is a semi-transparent film made ofmolybdenum silicide-based materials, a pattern may be formed bydry-etching with the use of fluorine-based gas such as CF₄, CHF₃ andC₂F₆, or mixed gas thereof, or alternatively a gas, with which oxygen ismixed, as etching gas. In the case where the semi-transparent film 92 ais made of a chromium-based material such as a chromium oxide nitridefilm, dry-etching may be performed with the use of mixed gas of Cl₂ andoxygen as etching gas.

Next, the above-mentioned first resist pattern 94 a is peeled off, and asecond resist pattern 94 b is formed on the light shielding film toremove the light shielding film 93 in the assist pattern part 96 byetching (FIG. 9C). Etching of the light shielding film 93 may bedry-etching and the light shielding film 93 may be also removed bywet-etching in an aqueous solution of a ceric ammonium nitrate salt.

Subsequently, the second resist pattern 94 b is peeled off, and thewhole principal plane of the transparent substrate 91 is dry-etched onthe etching conditions of the semi-transparent film 92 to form an assistpattern 98 by dry-etching the semi-transparent film of the assistpattern part until film thickness for allowing a predeterminedretardation within the scope of 70° to 115° between the lighttransmitting through the assist pattern and the light transmittingthrough a transparent region of the transparent substrate 91 is obtained(FIG. 9D). The etching amount of the assist pattern 98 for allowing theabove-mentioned retardation corresponds to a predetermined filmthickness difference within the scope of 24 nm to 40 nm in filmthickness difference from the main pattern. The main pattern part is notetched by reason of being covered with the light shielding film 93.

Next, the light shielding film 93 of the main pattern part is removed byetching to form a main pattern 99 and then a halftone mask 90 having theassist pattern 98 for generating a retardation of 180° between the lighttransmitting through the main pattern 99 and the light transmittingthrough a transparent region of the transparent substrate 91 (FIG. 9E).In the process of FIG. 9E, the light shielding film 93 may be removed byeither method of dry-etching or wet-etching.

The manufacturing method of a photomask according to the above-mentionedthird embodiment allows the high-quality halftone mask 90, such that thedigging depth of the transparent substrate is prevented from varying ina mask plane and between patterns, without causing the level differenceas described in FIGS. 11A to 11E on the transparent substrate 91surface.

Fourth Embodiment

FIGS. 10A to 10D are a process cross-sectional schematic view showing afourth embodiment of a manufacturing method of a photomask of thepresent invention. The fourth embodiment is a manufacturing method of aphotomask in the case of leaving a light shielding film in a necessarypredetermined spot in the above-mentioned first to third embodiments.

Ordinarily, the mask outer periphery is subject to multiple exposure ina projection exposure, so that a photomask provided with a lightshielding region in the mask outer periphery is used. The fourthembodiment is an example such that a light shielding region is providedin the photomask outer periphery, and is described by FIGS. 10A to 10Dwhile hereinafter referring to FIGS. 7A to 7E for the reason that theprocesses from the beginning to the halfway are the same as theprocesses described in the first to third embodiments. In FIGS. 10A to10D, the same reference numerals are used in the same spots as FIGS. 7Ato 7E.

As shown in FIG. 10A, an assist pattern part 108 is formed by performingthe production processes until the process shown in FIG. 7D. At thistime, a light shielding film in a predetermined spot necessary as aphotomask is previously left. FIGS. 10A to 10D exemplify the case ofleaving a light shielding film 104 as a light shielding region in thephotomask outer periphery.

Next, as shown in FIG. 10B, a resist pattern 105 for a light shieldingregion is formed on the light shielding film 104 in a necessarypredetermined spot. The resist pattern 105 for a light shielding regionmay be formed not merely on the light shielding film 104 but also so asto cover the assist pattern 108. Next, a light shielding film 103 on amain pattern is removed by etching (FIG. 10C), and subsequently theresist pattern 105 for a light shielding region is peeled off to form amain pattern 109 and then a halftone mask 100 having the assist pattern108, such that the light shielding film 104 as a light shielding regionis provided in the photomask outer periphery (FIG. 10D).

The manufacturing method of a photomask according to the above-mentionedfourth embodiment allows the high-quality halftone mask having theassist pattern, such that the light shielding region is provided in themask outer periphery, without causing the level difference as describedin FIGS. 11A to 11E on the transparent substrate 101 surface.

Also in the second and third embodiments as a manufacturing method of aphotomask of the present invention, a light shielding region may beprovided similarly in a desired region such as the mask outer periphery.

(SRAF Etching Amount and SRAF Dimension on Wafer)

Next, with regard to a manufacturing method of the present invention, anembodiment in changing a pitch in a line/space pattern is described indetail.

As described above, in order that SRAF may not be transferred on awafer, SRAF light intensity/slice level needs to be 1 or more. FIG. 12is a view showing a relation between SRAF etching amount (on a mask) andSRAF CD (a dimension on a wafer), which satisfies SRAF lightintensity/slice level=1.1 with a margin of 10% in the embodiment of aCquad illumination shown in FIGS. 3A and 3B. The SRAF etching amountcorresponds to retardation in an SRAF part and larger SRAF etchingamount brings larger SRAF dimension transferred on a wafer. The SRAFetching amount signifies film thickness difference between SRAF filmthickness after etching and film thickness of a main pattern (theinitial film thickness of a semi-transparent film: 68 nm).

In FIG. 12, a region with an SRAF etching amount of 48 nm or more shownby a dotted arrow in the FIG. corresponds to a region with an SRAF partretardation of 50° or less (the scope described in the invention of theabove-mentioned Patent Literature 2). In this case, SRAF CD on a waferbecomes 50 nm or more. However, when SRAF dimension on a wafer is 50 nm(200 nm on a quadruple mask) or more, a space between the main patternand SRAF becomes as narrow as 200 nm or less on a mask, which is astrict value for scarcely allowing misalignment in mask productionprocesses. In the present laser exposure device for mask production,ordinarily, 200 nm or more is so necessary as a space between patternsin consideration of misalignment that too large SRAF dimension makesmask production difficult. On the other hand, when an SRAF etchingamount is less than 24 nm (SRAF CD on a wafer is 20 nm), SRAF dimensionmay not sufficiently be enlarged. Accordingly, in FIG. 12, a regionshown by a solid double-pointed arrow is a preferable SRAF etchingamount region in consideration of mask production.

(Influence of SRAF Etching Amount Error on Main Pattern CD)

Next, in the case where an error occurs in an SRAF etching amount, aninfluence on main pattern CD adjacent to SRAF is described by FIG. 13.FIG. 13 shows main pattern CD error on a wafer with respect to etchingamount error when an SRAF etching amount is 28 nm, 38 nm and 48 nm inthe embodiment of a Cquad illumination shown in FIGS. 3A and 3B, and itis found that larger SRAF etching amount brings larger main pattern CDfluctuation on a wafer. When an SRAF etching amount is 48 nm, it isshown that a slight etching error has a great influence on main patterndimension at a repetition end. Accordingly, in the present invention, anSRAF etching amount of 48 nm or more (corresponding to a retardation of50° or less in Patent Literature 2) is an unpreferable scope inproduction processes.

(Influence on SRAF Etching Amount and Main Pattern at Repetition End)

When an SRAF etching amount is changed, the influence on main pattern CDat a repetition end and Defocus, and light intensity distribution aredescribed.

FIG. 14 is a view showing a relation between main pattern CD at arepetition end on a wafer and Defocus in changing an SRAF etching amountat every 4 nm within the scope of 24 nm to 48 nm in the embodiment of aCquad illumination shown in FIGS. 3A and 3B. For reference, the case ofno SRAF itself and the case of no SRAF etching are also shown in thedrawings. When an SRAF etching amount is within the scope of 24 nm to 40nm, the fluctuation of main pattern CD is comparatively gentle to thechange of Defocus and shows approximately the same behavior. However,when an SRAF etching amount is 44 nm and 48 nm, main pattern CD shows agreat fluctuation with respect to the change of Defocus.

FIG. 15 shows light intensity distribution of a main pattern at arepetition end on a wafer in changing an SRAF etching amount at every 4nm within the scope of 24 nm to 48 nm in the embodiment of a Cquadillumination shown in FIGS. 3A and 3B. When an SRAF etching amount iswithin the scope of 24 nm to 40 nm, the inclination of light intensitydistribution is comparatively large and shows approximately the samebehavior. However, when an SRAF etching amount is 44 nm and 48 nm, theinclination of light intensity distribution becomes so small as to showthat resolution of the main pattern decreases.

Accordingly, through the results shown in FIGS. 12 to 15, an SRAFetching amount of 44 nm or more is an inappropriate scope, and an SRAFetching amount of 24 nm to 40 nm is a preferable scope for improvingfocal depth and forming a high-resolution pattern. This etching amountcorresponds to a retardation of 115° to 70°, respectively. Themeasurement of retardation was performed by the above-mentioned phaseshift amount measuring apparatus (MPM193™: manufactured by LasertecCorporation).

(Verification in SRAF Between Main Patterns)

Next, the present invention is verified with regard to the case where anassist pattern (SRAF) exists between main patterns as anotherembodiment.

The same EM-Suite (trade name, manufactured by Panoramic TechnologyInc.) as the above was used as simulation software. The main simulationconditions were such that an ArF excimer laser (193 nm) was used as anilluminating source, NA was 1.35, and a Quasar (registered trademark)pupil filter 161 shown in FIGS. 16A to 16C was used. FIG. 16A is a planschematic view of the Quasar 161, FIG. 16B is a perspective schematicview in irradiating a mask 163 with exposing light by using the Quasar161 (referred to as a Quasar illumination), and FIG. 16C is a planschematic view of a mask pattern 164. The Quasar was such that anangular aperture of a fan-shaped light transmission part was 30°, anoutside diameter was 0.85, and an inside diameter was 0.65 (a radius ofthe pupil filter is regarded as 1). A molybdenum silicide-based halftonemask with a transmittance of 6% in an exposure wavelength of 193 nmhaving an assist pattern of the present invention (6%-halftone) was usedas the mask. A target CD on a wafer was 60 nm, an SRAF 166 existed ineach space between main patterns 165, a pattern pitch was a throughpitch line/space from the minimum pitch of 120 nm, and the SRAF 166 wasof a pitch of 250 nm.

FIG. 17 is a view showing a relation between SRAF etching amount (on amask) and SRAF CD (a dimension on a wafer) in the embodiment of a Quasarillumination shown in FIGS. 16A to 16C. In FIG. 17, similarly to FIG.12, a region with an SRAF etching amount of 48 nm or more shown by adotted arrow in the drawing corresponds to a region with an SRAFretardation of 50° or less (the scope described in the invention of theabove-mentioned Patent Literature 2). In the case of the presentembodiment, as compared with the conditions of a main pattern at aline/space repetition end and Cquad shown in FIG. 12, an original SRAFdimension is so small as 9 nm on a wafer (36 nm on a mask) that theproblem that the SRAF dimension on a wafer is too large is not causedeven in a region with an SRAF etching amount of 48 nm or more.

FIG. 18 is a view showing an influence on main pattern CD on a wafer inthe case where an error occurs in an SRAF etching amount on a mask inthe embodiment of a Quasar illumination shown in FIGS. 16A to 16C.Similarly to FIG. 13, the case where an SRAF etching amount is 28 nm, 38nm and 48 nm is shown. As shown in FIG. 18, main pattern CD error on awafer is extremely small with respect to SRAF etching amount error.

FIG. 19 is a view showing a relation between main pattern CD and Defocusin changing an SRAF etching amount at every 4 nm within the scope of 24nm to 48 nm similarly to FIG. 14 in the embodiment of a Quasarillumination shown in FIGS. 16A to 16C. For reference, the case of noSRAF and the case of no SRAF etching are also shown in the drawings.

As shown in FIG. 19, focal depth is increased by providing SRAF withrespect to no SRAF as shown by a solid arrow in the drawing. However,even in the case of no SRAF etching, the main pattern CD is asymmetricwith respect to Defocus. As shown by a dotted arrow in the drawing,larger SRAF etching amount brings more asymmetry with respect toDefocus; the main pattern CD on a wafer rises on the minus side ofDefocus and the main pattern CD on a wafer lowers on the plus side ofDefocus, and fluctuation of dimension of the main pattern on a waferbecomes asymmetric. For example, when an SRAF etching amount isincreased to 48 nm, transferred image properties are deteriorated due tothe asymmetry property. Through the results shown in FIGS. 17 to 19, inthe present invention, the upper limit of an SRAF etching amount wasdetermined at 40 nm. Accordingly, also in the case of SRAF between themain patterns (a Quasar illumination), the effect shown by a photomaskof the present invention was verified similarly to SRAF at a mainpattern repetition end (a Cquad illumination).

As described above, with regard to a manufacturing method of a photomaskof the present invention, a main pattern and an assist pattern are eachconstituted from a semi-transparent film made of the same material, sothat a film-forming process of the semi-transparent film is easy. Also,retardation between the light transmitting through the assist patternand the light transmitting through a transparent region of a transparentsubstrate is determined at a predetermined retardation within the scopeof 70° to 115°, and the semi-transparent film of the assist pattern isdry-etched to obtain film thickness difference between a first thicknessof the main pattern and a second thickness of the assist pattern as apredetermined film thickness difference within the scope of 24 nm to 40nm, namely, an etching amount of the assist pattern, so that a desiredretardation of the assist pattern may be easily obtained. In addition, aspace between the main pattern and the assist pattern is widened so moreas to allow a manufacturing method having a high margin of misalignmentand allow a photomask for improving transfer properties of the patternwithout increasing difficulty in mask production.

C. Photomask

A photomask of the present invention is a photomask comprising on aprincipal plane of a transparent substrate a main pattern which istransferred to a transfer-target surface by the above-mentionedprojection exposure and an assist pattern which is formed nearby theabove-mentioned main pattern and not transferred to the above-mentionedtransfer-target surface, characterized in that the above-mentioned mainpattern and the above-mentioned assist pattern are each constituted froma semi-transparent film made of the same material, film thickness of theabove-mentioned assist pattern is thinner than that of theabove-mentioned main pattern, and film thickness difference is apredetermined film thickness difference within the scope of 24 nm to 40nm.

According to a photomask of the present invention, the determination offilm thickness difference between a first thickness of the main patternand a second thickness of the assist pattern at a predetermined scopeallows phenomena to be restrained from occurring such that part of theassist pattern is chipped, the assist pattern is peeled off from asubstrate surface, and the assist pattern falls in the line widthdirection.

Examples of a photomask of the present invention include the samephotomask as the above-mentioned photomask shown in FIGS. 1 and 2.

Also, a photomask of the present invention preferably uses a shortwaveexposing source as an exposing source. Examples of such a shortwaveexposing source include excimer lasers such as an ArF excimer laser anda KrF excimer laser, and an i line of a mercury-vapor lamp; among them,excimer lasers are preferable and an ArF excimer laser is particularlypreferable.

In addition, a photomask of the present invention may be a photomaskused for an exposure by an ordinary illumination, or a photomask usedfor a projection exposure by an off axis illumination. The descriptionof members of the photomask and other technical characteristics is thesame as the contents described in the above-mentioned “A. Photomask” andthe above-mentioned “B. Manufacturing method of photomask”; therefore,the description herein is not repeated here.

D. Correcting Method of Photomask

A photomask intended for by a correcting method of a photomask of thepresent invention is a mask having an assist pattern, which uses an ArFexcimer laser as an exposing source, is used for a projection exposureby an off axis illumination, and is preferably intended to be used forforming a minute semiconductor device with a half pitch of 65 nm orless, further 45 nm and 32 nm on a wafer.

(Transfer Properties of Photomask Having Assist Pattern)

The transfer properties of a photomask having an assist pattern arefirst described while taking a halftone mask as an example beforedescribing a correcting method of the present invention. The inventorsof the present invention have examined the transfer properties of ahalftone mask having an assist pattern for forming a minute pattern witha half pitch of 45 nm or less on a wafer by simulation while comparingwith a binary mask.

In the simulation, EM-Suite (trade name, manufactured by PanoramicTechnology Inc.) was used as simulation software for estimating thetransfer properties of a mask pattern. The main simulation conditionsare the same as the above-mentioned contents described by using FIGS. 3Aand 3B.

FIGS. 4A and 4B is an evaluation pattern used for the simulation (FIG.4A), and a view of an aerial image showing light intensity correspondingto a position of the evaluation pattern (FIG. 4B). The contents of theevaluation pattern are the same as the above-mentioned contentsdescribed by using FIGS. 4A and 4B.

In a halftone mask having the above-mentioned assist pattern, transferproperties of the assist pattern (SRAF) at the ends of the line/spacepattern are as described above.

FIG. 23 is a view showing a ratio (the vertical axis) of SRAF part lightintensity to slice level of normalized light intensity threshold withrespect to SRAF CD (the horizontal axis) on a wafer in a halftone maskand a binary mask in the case where film thickness of the main patternand the assist pattern (SRAF) obtained by the above-mentioned simulationis the same. The halftone mask (a triangular point in the drawing) showsthe case where the main pattern CD is of three kinds (32 nm; 36 nm; and40 nm on a wafer). The above-mentioned ratio of 1 or less causes SRAF tobe transferred, so that the above-mentioned ratio needs to be a valuemore than 1 for not transferring SRAF. When the main pattern CD of thehalftone mask shown by a clotted line in the drawing is 32 nm (128 nm onthe mask), SRAF is resolved if SRAF CD is not 14 nm (56 nm on the mask)or less.

The above is a simulation result in the case of using the 6%-halftonemask having SRAF, and when the main pattern CD of the halftone mask isas minute as 32 nm, it is found that the SRAF dimension becomes soextremely small that the actual mask production is difficult.

<Correcting Method of Photomask of the Present Invention>

Next, embodiments of a correcting method of a photomask of the presentinvention are described in detail on the basis of drawings whilereferring to the above-mentioned results. In describing transferproperties of the following mask pattern, a Cquad pupil filter 31 shownin the above-mentioned FIGS. 3A and 3B was used and EM-Suite (tradename, manufactured by Panoramic Technology Inc.) was used as simulationsoftware. The main simulation conditions are such that an ArF excimerlaser (193 nm) is used as an illuminating source and NA is 1.35. Thepattern shown in the above-mentioned FIG. 4A is used for an evaluationpattern.

FIGS. 25A to 25C are cross-sectional schematic views showing a processoutline of an embodiment of a correcting method of a photomask having anassist pattern of the present invention, and exemplifies the case of ahalftone mask provided with a line/space pattern. FIG. 25A is across-sectional schematic view of a photomask before being corrected,and is a halftone mask 10 in which a main pattern 12 for transmittingexposing light at a predetermined transmittance to change a phase isprovided on a transparent substrate 11 such as a synthetic quartzsubstrate, the main pattern 12 is composed of a single-layersemi-transparent film 14, and an assist pattern (SRAF) 13 composed of asemi-transparent film with the same film thickness made of the samematerial as the main pattern 12 is formed nearby the main pattern 12.FIGS. 25A to 25C exemplify only two lines of the main pattern 12 and theassist pattern 13 each and part of a mask pattern, and needless to say,is not limited thereto. The main pattern may be an isolated pattern or aperiodic pattern.

The halftone mask 10 having an assist pattern of the present embodimentis determined so that a retardation of 180° is generated between theexposing light transmitting through the main pattern 12 and the exposinglight transmitting through a transparent region with no patterns of thetransparent substrate 11. The measurement of retardation may beperformed by a phase shift amount measuring apparatus (such as MPM193™:manufactured by Lasertec Corporation).

Here, the halftone mask 10 shown in FIG. 25A is a mask which uses an ArFexcimer laser as an exposing source, in which assist patterns 13 a and13 b nearby the main pattern 12 are resolved on a wafer of atransfer-target surface when the mask pattern is transferred on a waferby a projection exposure by an off axis illumination.

FIG. 25B is a cross-sectional schematic view showing a state duringcorrection of a photomask in which the above-mentioned assist patterns13 a and 13 b are resolved on a wafer. The above-mentioned assistpatterns 13 a and 13 b to be resolved on a transfer-target surface areof properties different from an unnecessary excessive defect notoriginally allowed to be on the mask, namely, called “black defect”, andare an indispensable region to the formation of the mask pattern on awafer. The assist patterns 13 a and 13 b to be resolved on atransfer-target surface on the mask may not be detected as a defect by aconventional mask defect inspection device for detecting a black defect.The detection of the assist patterns 13 a and 13 b to be resolved may beperformed by verification with the use of a lithographic simulationmicroscope such as Aerial Image Measurement System (manufactured by CarlZeiss, abbreviated as AIMS (registered trademark), also referred to asAIMS hereinafter), and an exposure test with an actual exposure device.

In a correcting method of the present invention, a surface in acorresponding region of the assist patterns 13 a and 13 b to be resolvedon a transfer-target surface is etched or ground to thin the filmthickness in a corresponding region of the assist patterns 13 a and 13 buntil the assist patterns 13 a and 13 b are not resolved on a wafer as atransfer-target surface. FIG. 25B exemplifies the case of correcting byan electron beam mask correcting device in such a manner that a regionof the assist pattern 13 a to be resolved on a wafer is etched to thinthe film thickness.

In the correction, the whole assist pattern plane of a line or plurallines does not always need to be etched to form a thin film, and only aregion of the assist pattern to be resolved may be etched to thin thefilm thickness. Needless to say, in the case where the whole assistpattern plane of a line is transferred, the film thickness of the wholeplane of a line may be thinned, and in the case where the whole assistpattern plane of plural lines is transferred, the film thickness of thewhole plane of plural lines may be thinned.

In a process of etching or grinding a surface of the above-mentionedassist patterns 13 a and 13 b to be resolved on a transfer-targetsurface, film thickness to be removed by etching or grinding the assistpattern 13 a may be previously obtained by simulation.

FIG. 25C is a cross-sectional schematic view showing a state of aphotomask after being corrected, in which a surface of the assistpatterns 13 a and 13 b to be resolved on a transfer-target surface isetched to thin the film thickness and obtain assist patterns 13 a′ and13 b′. The correction spot becomes thinner through etching by filmthickness difference T than film thickness before being corrected.

In the present invention, various kinds of methods conventionally usedfor correcting a black defect on a photomask may be applied to a methodof thinning film thickness of a region of the above-mentioned assistpattern to be resolved on a transfer-target surface. Examples thereofinclude a gas-assisted etching method by using an ion beam of a focusedion beam (FIB) mask correcting device, a gas-assisted etching method byusing an electron beam (EB) of an electron beam (EB) mask correctingdevice as shown in the above-mentioned FIG. 25B, a method of physicallygrinding an assist pattern having a defect by using a probe of an atomicforce microscope (AFM), or a method in such a manner that a resistpattern is formed on a mask to expose only a defective region of anassist pattern and thin the film thickness of the defective region isselectively obtained by dry-etching.

However, with regard to the gas-assisted etching method by an FIB maskcorrecting device among the above-mentioned methods, a phenomenon of agallium stain such that gallium ordinarily used as an ion beam is driveninto a transparent substrate causes light transmittance in a correctionsite to be decreased, or a digging phenomenon of a transparent substratecalled a river bed by overetching occurs easily in a transparentsubstrate on the periphery of a correction site. The method of forming aresist pattern requires processes of resist application, pattern drawingand resist peeling, and causes a problem such that a correction processis extended.

On the other hand, the gas-assisted etching method by an EB maskcorrecting device is, for example, as shown in FIG. 25B, a method insuch a manner that assist gas most suitable for etching is dischargedfrom a gas nozzle 15 nearby an electron beam 16 for intensively scanningthe assist pattern 13 a having a defect, a molecule of the gas adheresto the assist pattern 13 a surface to be corrected, and a chemicalreaction is caused by the electron beam to perform etching whilechanging the assist pattern material into a volatile substance, whichmethod is suitable for a minute pattern and does not damage a correctionspot. The confirmation of a correction spot is performed by SEM providedfor an EB mask correcting device. Examples of the above-mentioned EBmask correcting device include MeRiT 65™ (manufactured by Carl Zeiss).

The method of grinding with a probe of AFM is a method of directlyshaving off a defect while applying a certain load to a hard probe suchas a diamond needle, in which the probe is set at the tip of acantilever to control the cantilever by using the principle of an atomicforce microscope. The confirmation of a correction spot is performed bySEM provided for AFM, and the method is such that the assist patternsurface having a defect to be corrected is scanned by the probe tosubsequently shaving off the defect itself with the probe. The method issuitable for correcting a minute pattern and the case where area and thefilm thickness to be removed are small. Examples of the above-mentionedmask correcting device with the use of a probe of AFM include RAVEnm650™ (manufactured by RAVE LLC).

Accordingly, in the present invention, a method of thinning the filmthickness of the assist pattern with a minute pattern formed is morepreferably the above-mentioned gas-assisted etching method by an EB maskcorrecting device or method of grinding with a probe of AFM.

In the present invention, as shown in FIG. 25C, film thicknessdifference T (SRAF film thickness difference in the drawing) between thefilm thickness of the assist patterns 13 a′ and 13 b′ after beingcorrected by etching or grinding to thin and the film thickness of theassist patterns 13 a and 13 b before being corrected is preferablywithin the scope of 1 nm to 40 nm. The film thickness may be measured byan atomic force microscope (AFM).

The assist pattern is originally designed and produced into such a maskas not to be resolved on a transfer-target surface, so that the assistpattern to be transferred on a wafer is frequently part thereof and thefilm thickness to be corrected is frequently very thin. Accordingly, thefilm thickness to be corrected is occasionally slight and the lowerlimit of film thickness to be corrected is determined at correctable 1nm in the present invention. The reason therefor is that the filmthickness to be corrected of less than 1 nm does not allow the effect ofcorrecting to be confirmed. On the other hand, the upper limit of filmthickness to be corrected is determined at 40 nm in the properties ofthe assist pattern. The reason therefor is that the film thickness to becorrected of more than 40 nm reduces or extinguishes original SRAFfunctions such as the effect of improving resolution of the main patternand the focal depth magnification effect by SRAF.

(Photomask to which Correcting Method of the Present Invention isApplicable)

A correcting method of a photomask of the present invention may be usedfor either mask of a halftone mask and a binary mask if they have anassist pattern, is not particularly limited, and is described whileexemplifying a typical mask of a halftone mask and a binary mask havingan assist pattern in FIGS. 26A to 26F. In FIGS. 26A to 26F, the samereference numerals are used in the case of showing the same site.Needless to say, a correcting method of a photomask of the presentinvention is not limited to the photomask shown in FIGS. 26A to 26F.

FIGS. 26A to 26D are partial cross-sectional schematic views showing anexample of a halftone mask, such that a main pattern and an assistpattern are composed of a semi-transparent film, to which a correctingmethod of the present invention is applicable. With regard to thephotomasks of FIGS. 26A to 26D, a main pattern 42 and an assist pattern43 are provided on a transparent substrate 41, and in any mask, filmthickness of the main pattern 42 is determined so as to generate aretardation of 180° between the exposing light transmitting through themain pattern 42 and the exposing light transmitting through atransparent region of the transparent substrate 41.

In addition, FIG. 26A is a halftone mask such that the main pattern 42and the assist pattern 43 are composed of the same semi-transparent filmwith the same film thickness. FIG. 26B is a mask such that the mainpattern 42 and the assist pattern 43 are composed of the samesemi-transparent film, and a predetermined retardation within the scopeof 70° to 115° is generated between the exposing light transmittingthrough the assist pattern 43 and the exposing light transmittingthrough a transparent region of the transparent substrate 41, in whichthe film thickness of the assist pattern 43 is thinner than that of themain pattern 42. FIG. 26C is a mask such that the main pattern 42 iscomposed of two layers of a transparent film/a semi-transparent film,and the assist pattern 43 is composed of a semi-transparent film, inwhich the main pattern 42 and the assist pattern 43 are the same in thefilm thickness of the semi-transparent film layer. FIG. 26D is a masksuch that the main pattern 42 is composed of two layers of asemi-transparent film/a semi-transparent film, and the assist pattern 43is composed of a semi-transparent film, in which the main pattern 42 andthe assist pattern 43 are the same in film thickness of thesemi-transparent film layer contacting with the transparent substrate41.

The above-mentioned masks having the assist pattern shown in FIGS. 26Ato 26D are typical examples, and examples of a mask to which acorrecting method of the present invention is applicable include a masksuch that the main pattern and the assist pattern are composed of twolayers of a semi-transparent film/a semi-transparent film.

FIGS. 26E and 26F are partial cross-sectional schematic views showing anexample of a binary mask such that a main pattern is composed of a lightshielding film for shielding exposing light. FIG. 26E is a mask suchthat the main pattern 42 is composed of two layers of a light shieldingfilm/a semi-transparent film on the transparent substrate 41, and theassist pattern 43 is composed of a semi-transparent film, in which themain pattern 42 and the assist pattern 43 are the same in film thicknessof the semi-transparent film layer. FIG. 26F is a mask such that themain pattern 42 and the assist pattern 43 are composed of the same lightshielding film with the same film thickness.

As described above, a semi-transparent film of the main pattern and theassist pattern in the photomask to which a correcting method of thepresent invention is applied signifies a semi-transparent thin film fortransmitting exposing light at a predetermined transmittance, which thinfilm may be a semi-transparent single-layer film, or a constitution of atwo-layer film or more of a semi-transparent film and a transparent filmor another semi-transparent film different in transmittance. Also, alight shielding film of the main pattern and the assist pattern in thephotomask to which a correcting method of the present invention isapplied signifies a thin film for shielding exposing light, which thinfilm may be a single-layer film of a light shielding film, or aconstitution of a two-layer film or more having a light shielding filmand a semi-transparent film.

In a correcting method of a photomask of the present invention, amaterial for a semi-transparent film composing the main pattern 42 andthe assist pattern 43 of the masks shown in FIGS. 26A to 265 is notparticularly limited, and examples thereof include semi-transparentfilms such as a molybdenum silicide oxide film (MoSiO), a molybdenumsilicide nitride film (MoSiN) and a molybdenum silicide oxide nitridefilm (MoSiON), which are molybdenum silicide-based materials,semi-transparent films such as a chromium oxide film (CrO), a chromiumnitride film (CrN) and a chromium oxide nitride film (CrON), which arechromium-based materials, and a semi-transparent film such as tin oxide(SnO₂). The molybdenum silicide-based semi-transparent films are put topractical use as a halftone mask material and are more preferablematerials. Examples of a transparent film composing the main pattern 42shown in FIG. 26C include a silicon oxide film (SiO₂). Examples of alight shielding film composing the main pattern 42 shown in FIG. 26E anda light shielding film composing the main pattern 42 and the assistpattern 43 of the mask shown in FIG. 26F include a metal thin film suchas a chromium film (Cr) and a metal silicide thin film such asmolybdenum silicide (MoSi).

In the gas-assisted etching process by using an electron beam of theelectron beam mask correcting device shown in FIG. 25B, in the casewhere the semi-transparent film composing the main pattern 42 and theassist pattern 43 is a semi-transparent film made of molybdenumsilicide-based materials, film thickness of the assist pattern may beselectively thinned by selectively etching with the use offluorine-based gas such as CF₄, CHF₃ and C₂F₆, or mixed gas thereof, orgas, with which oxygen is mixed, as assist gas. In the case where thelight shielding film composing the main pattern 42 and the assistpattern 43 is made of chromium, film thickness of the assist pattern maybe selectively thinned by selectively etching with the use of mixed gasof Cl₂ and oxygen as assist gas.

(Transfer Properties of Assist Pattern after being Corrected)

Next, the effect of thin-film forming by a correcting method of aphotomask of the present invention shown in FIGS. 25A to 25C, such thatan assist pattern (SRAF) surface is etched or ground to thinly correctthe film thickness of the assist pattern, is described. The mask isdescribed while taking a halftone mask in the shape of a partialcross-sectional schematic view shown in FIG. 26A as an example, suchthat molybdenum silicide with a film thickness of 68 nm is used as asemi-transparent film, a main pattern (a film thickness of 68 nm) hasArF excimer laser light (193 nm) transmittance of 6% and a retardationof 180° with a transparent region of a transparent substrate, and filmthickness of the assist pattern before being corrected is also 68 nm.

EM-Suite (trade name, manufactured by Panoramic Technology Inc.) wasused as simulation software. The main simulation conditions were suchthat an ArF excimer laser (193 nm) was used as an illuminating source,NA was 1.35, and a Cquad pupil filter was used as an off axisillumination; a Cquad 21 was such that an angular aperture of afan-shaped light transmission part was 35°, an outside diameter was 0.9,and an inside diameter was 0.7 (a radius of the pupil filter is regardedas 1). The mask adopted values of the above-mentioned halftone mask.

FIG. 5 is a result obtained by the simulation and a view showing arelation between SRAF film thickness difference (the horizontal axis)and SRAF light intensity/slice level of normalized light intensitythreshold (the vertical axis) in changing CD of SRAF in theabove-mentioned halftone mask such that CD of the main pattern on awafer is 32 nm. In FIG. 5, it is shown that SRAF is resolved on a waferunless SRAF light intensity/slice level is 1 or more.

As shown in FIG. 5, when CD of SRAF is as minute as 14 nm (56 nm on amask), even though SRAF film thickness difference after being correctedis the same as 0 (the same as the case of not being corrected), SRAFlight intensity/slice level is 1 or more and SRAF is not essentiallytransferred. Next, when CD of SRAF is 22 nm (88 nm on a mask), it isshown that SRAF is not resolved and transferred if SRAF film thicknessdifference after being corrected is 24 nm or more. Similarly, when CD ofSRAF is 26 nm (104 nm on a mask), SRAF is not transferred if SRAF filmthickness difference after being corrected is 30 nm or more, and when CDof SRAF is 30 nm (120 nm on a mask), SRAF is not transferred if SRAFfilm thickness difference after being corrected is 34 nm or more.

As described in the above-mentioned FIGS. 5 and 23, in a halftone masksuch that the main pattern and the assist pattern (SRAF) are composed ofthe same material and the same film thickness, when CD of the mainpattern is determined at 32 nm, SRAF is used only if CD of SRAF is 14 nmor less; however, as described above, the application of a correctingmethod of the present invention such that SRAF to be transferred isformed into a thin film allows SRAF to be also used without beingresolved and transferred even though CD of SRAF is enlarged toapproximately twice large dimension, that is, 26 nm to 30 nm. Acorrecting method of the present invention extends the availability of ahalftone mask having SRAF which is conventionally difficult to bemicronized and used.

Next, the influence in thinning SRAF by correcting is described. FIG. 6is a view showing a relation between line CD and Defocus (focal positionchange) at a main pattern end on a wafer in changing CD of SRAF obtainedby the simulation. The film thickness difference from the film thicknessbefore being corrected (SRAF film thickness difference: 24 nm, 32 nm and40 nm) is offered by correcting through etching to thin the filmthickness so that SRAF is not resolved with respect to each CD of SRAF.As shown in FIG. 6, CD of SRAF is enlarged to 22 nm to 30 nm (on awafer) to perform correction for thinning the film thickness of SRAF, sothat approximately the same tendency is exhibited without any CD changeamong each of SRAF dimensions when the focus is changed. That is to say,it is shown that the formation of SRAF into a thin film by a correctingmethod of the present invention has no bad influence on Defocus to allowthe same dimensional accuracy as the case of uncorrected SRAF CD of 14nm.

In the above-mentioned embodiments, the photomask having the assistpattern (SRAF) was described while taking a mask aspect having SRAF atboth ends of the main pattern as an example; yet, the present inventionis not limited thereto and a correcting method of a photomask of thepresent invention may be also applied to a mask aspect having SRAFbetween the main patterns or a mask aspect having an isolated pattern asthe main pattern.

According to a correcting method of a photomask of the presentinvention, in a correcting method of a photomask in the case where anassist pattern is resolved and transferred on a transfer-target surface,a surface of the assist pattern is etched or ground to thin the filmthickness of the assist pattern until the assist pattern is not resolvedon a transfer-target surface, so that a problem such that the assistpattern is resolved and transferred may be solved to correct to aphotomask for forming a transferred image having high contrast whilekeeping the focal depth magnification effect as the assist pattern. Thecorrecting method of a photomask of the present invention differs from aconventional correcting method of a photomask such that a mask patternis corrected in the line width direction of the assist pattern, is amethod of correcting a mask pattern in the thickness direction of theassist pattern, and allows a photomask having the assist pattern to becorrected by a sure and comparatively easy method.

E. Corrected Photomask Corrected Photomask of the Present Invention

A corrected photomask of the present invention is a photomask such thatan assist pattern is corrected by the above-mentioned correcting methodof a photomask, as an example, which is provided with the assistpatterns 13 a′ and 13 b′ after being corrected by etching or grinding tothin, and has the film thickness difference (SRAF film thicknessdifference in the drawing: T) from film thickness of the assist patternsbefore being corrected, as shown in FIG. 25C. In a photomask of thepresent invention, with regard to the photomask in which an assistpattern is resolved and transferred to a transfer-target surface, theassist pattern is corrected in the thickness direction of filmthickness, so that the assist pattern is not resolved and transferred toa transfer-target surface and a transferred image having high contrastmay be formed while keeping the focal depth magnification effect.

The present invention is hereinafter described by examples.

Examples

An MoSi-based halftone mask having an assist pattern and a transmittanceof 6% at 193 nm was produced as a mask for an ArF excimer laser (awavelength of 193 nm). A pattern shown in FIGS. 4A and 4B was formed onthe conditions that a target line dimension on a wafer was 45 nm and thepattern was a line/space repetition pattern with a pitch of 90 nm (apitch of 360 nm on the mask). Nine line/space with a half pitch of 45 nmas a main pattern and two SRAF (the pitch of SRAF is 90 nm) at both endsof the main pattern for improving resolution of the main pattern at theends are put on a wafer. Both the main pattern and SRAF were composed ofthe above-mentioned 6%-halftone and film thickness of both patterns onthe mask was determined at 68 nm. Both CD of the main pattern and CD ofSRAF on the mask were determined at 128 nm.

ArF excimer laser exposure was performed by using the above-mentionedhalftone mask. NA in the exposure system was 1.35 and a Cquad pupilfilter shown in FIGS. 3A and 3B was used as an off axis illumination.However, the problem was caused such that the SRAF pattern, which maynot be transferred on a wafer, was resolved on a wafer.

Thus, in order to etch or grind SRAF to thin the film thickness thereof,transfer properties were previously estimated by simulation. Thetransfer properties to a wafer in thinning the film thickness of SRAFwere verified by using a lithographic simulation microscope AIMS45-193i™(manufactured by Carl Zeiss) on the same exposure conditions as theabove-mentioned exposure system.

First, the etching conditions of SRAF were confirmed by a test sample.The pattern was a line/space pattern with a pitch of 360 nm on the mask(90 nm on a wafer), and two SRAF were provided at both ends of the mainpattern. FIG. 27 is an SEM plane photograph after gas-assisted etchingwas performed by using CF₄ as assist gas with the use of an EB maskcorrecting device MeRiT 65™ (manufactured by Carl Zeiss) to partiallyetch an SRAF part (S1 and S2) of an MoSi thin film on a quartz substrateby 30 nm. A region inside the broken line of FIG. 27 is a part formedinto a thin film by etching, and a slight difference is microscopicallyconfirmed in the SEM photograph between the etched region and theunetched region, which etched region shows a favorable surface state.

FIG. 28 is a verification image by a lithographic simulation microscopeAIMS (AIMS45-193i™, manufactured by Carl Zeiss) after partially etchingan SRAF part (S1 and S2) by 30 nm in the above-mentioned test sample,and shows a planar state of light intensity distribution on a wafer. Aregion inside the broken line of FIG. 28 corresponds to the SRAF partformed into a thin film by etching, and it is shown that light intensityof this part increases and the resist pattern is not resolved.

The verification results of transfer properties by simulation with theuse of AIMS are shown in FIGS. 29 to 34. FIG. 29 is a view of an aerialimage showing light intensity corresponding to a position of the maskpattern before thinning SRAF, and SRAF (S1, S2) on one side of a pair ofpattern both ends and part of the main pattern are shown. In FIG. 29,the horizontal axis shows a position of part of the main pattern and apair of the pattern of SRAF, and the vertical axis shows normalizedlight intensity when light intensity of a transmission part with nopatterns is regarded as 1. The indication of plural light intensityprofiles shows the case of changing focus (focal depth) for watching theeffect of SRAF. As shown in FIG. 29, it was shown that S1 of SRAF wasresolved on a wafer even though slice level was any within the scope of0.25 to 0.57 in light intensity, and S2 was also resolved when slicelevel was 0.4 or more.

FIG. 30 is a partial magnified view of FIG. 29 and a view of an aerialimage showing a relation between a pattern position and light intensityof the main pattern at a repetition end and the assist pattern S1 beforeforming SRAF into a thin film in changing focus. The light intensitythreshold such that CD of the main pattern of a line/space repetitionpart was 45 nm was regarded as 0.42. In FIG. 30, as described in FIG.29, five light intensity profiles in the case of changing focus forwatching the effect of SRAF are shown. As shown in FIG. 30, S1 of SRAFis resolved regardless of the focus.

Here, FIG. 31 is a view of an aerial image showing a relation between apattern position and light intensity of the main pattern at a repetitionend and the assist pattern S1 in the case of no SRAF, as a comparisonreference of FIG. 30. As shown in FIG. 31, in the case of no SRAF, themain pattern at a repetition end is scarcely resolved.

Next, transfer properties in the case of forming SRAF into a thin filmby correcting were estimated by simulation. FIG. 32 is an estimationresult of transfer properties by simulation and a view of an aerialimage showing light intensity corresponding to a position of the maskpattern after thinning SRAF by 30 nm. The formation of SRAF into a thinfilm increased light amount by SRAF to increase the minimum value oflight intensity of SRAF; it was shown that the SRAF pattern was notresolved within the scope of 0.25 to 0.55, and the margin for selectingslice level of light intensity extended. It was confirmed that theoptical image of the main pattern did not deteriorate even though SRAFwas formed into a thin film.

FIG. 33 shows a situation of FIGS. 27 and 28 inside a broken line, andis a partial magnified view of FIG. 32 and a view of an aerial imageshowing a relation between a pattern position and light intensity of themain pattern at a repetition end and the assist pattern S1 after formingSRAF into a thin film. In FIG. 33, light intensity profiles in the caseof changing focus for watching the ef of SRAF are shown. As shown inFIG. 33, the formation of SRAF into a thin film allows S1 of SRAF not tobe resolved.

FIG. 34 is a view showing focal depth of the main pattern at arepetition end after forming SRAF into a thin film. As described above,the light intensity [a.u.] threshold such that CD of the main pattern ofa line/space repetition part was 45 nm was regarded as 0.42. Forcomparison, the case of no SRAF is also shown in the drawing. It wasshown that the focus scope to be resolved was narrow in the case (no) ofno SRAF and the focus scope to be resolved was wide in the case (thin)of providing SRAF formed into a thin film. Also, it was shown that theeffect as SRAF was kept while not resolving SRAF.

The SRAF part of the above-mentioned halftone mask having the SRAFpattern to be resolved on a wafer was corrected on the basis of theabove-mentioned etching test results and simulation results. The wholesurface of two SRAF on each side of a pair of the main pattern ends wasetched on the same etching conditions as the above-mentioned test sampleand formed into a thin film until the film thickness difference from theinitial film thickness became 30 nm.

When ArF excimer laser exposure was performed again by using thehalftone mask corrected by thinning film thickness of theabove-mentioned SRAF, SRAF was not resolved at all and only the mainpattern was transferred with high resolution on a wafer. Also, the focaldepth magnification effect during exposure was obtained.

REFERENCE SIGNS LIST

-   -   10, 20 halftone mask    -   11, 21 transparent substrate    -   12, 22 main pattern    -   13, 13 a, 13 a′, 13 b, 13 b′, 23 assist pattern (SRAF)    -   14 semi-transparent film    -   15 gas nozzle    -   16 electron beam    -   24 lower-layer semi-transparent film (etch stop layer)    -   25 upper-layer semi-transparent film    -   31, 161 pupil filter    -   32, 162 illuminating light    -   33, 163 mask    -   41 transparent substrate    -   42 main pattern    -   43 assist pattern (SRAF)    -   164 mask pattern    -   165 main pattern    -   166 SRAF    -   70, 80, 90, 100 halftone mask    -   71, 81, 91, 101 transparent substrate    -   72, 82, 102 semi-transparent film    -   73, 83, 93, 103 light shielding film    -   74, 84 first resist pattern    -   75, 85, 95 main pattern part    -   76, 86, 96 assist pattern part    -   77, 87 second resist pattern    -   78, 88, 98 assist pattern    -   79, 89, 99 main pattern    -   92 a lower-layer semi-transparent film (etch stop layer)    -   92 upper-layer semi-transparent film    -   94 a first resist pattern    -   94 b second resist pattern    -   94 c third resist pattern    -   104 light shielding film    -   105 resist pattern for light shielding region    -   110 halftone mask of conventional manufacturing method    -   111 transparent substrate    -   112 semi-transparent film    -   113 light shielding film    -   114 first resist pattern    -   115 main pattern part    -   116 assist pattern part    -   117 second resist pattern    -   118 assist pattern    -   119 main pattern    -   121 level difference on transparent substrate surface    -   1 main pattern    -   2 semi-transparent assist pattern    -   301 transparent substrate    -   302 semi-transparent film    -   304 transparent film

The invention claimed is:
 1. A correcting method of a photomask using anArF excimer laser as an exposing source, being used for a projectionexposure by an off axis illumination, and comprising on a principalplane of a transparent substrate a main pattern transferred to atransfer-target surface by the projection exposure and an assist patternformed nearby the main pattern, in which a minimum pattern pitch of themain pattern on the transfer-target surface is 120 nm or less, in a casewhere the assist pattern is resolved on the transfer-target surface bythe projection exposure; wherein a surface of the assist pattern to beresolved is etched to make a film thickness of the assist pattern to beresolved thinner than a film thickness of the main pattern until theassist pattern is not resolved on the transfer-target surface.
 2. Thecorrecting method of a photomask according to claim 1, wherein a filmthickness difference between a film thickness of the assist patternafter being corrected by etching to thin and a film thickness of theassist pattern before being corrected is within a scope of 1 nm to 40nm.
 3. The correcting method of a photomask according to claim 1,wherein the etching is a gas-assisted etching by using an electron beamof an electron beam mask correcting device.
 4. The correcting method ofa photomask according to claim 1, wherein the main pattern and theassist pattern are each constituted from a semi-transparent film, and afilm thickness of the main pattern is a film thickness for generating aretardation of 180° between a light transmitting through the mainpattern and a light transmitting through a transparent region of thetransparent substrate.
 5. The correcting method of a photomask accordingto claim 1, wherein the main pattern is constituted from a lightshielding film, and the assist pattern is constituted from asemi-transparent film.
 6. The correcting method of a photomask accordingto claim 1, wherein the main pattern and the assist pattern are eachconstituted from a light shielding film.
 7. The correcting method of aphotomask according to claim 1, wherein both the main pattern and theassist pattern are line patterns, and the main pattern is an isolatedpattern or a periodic pattern.
 8. A photomask wherein the assist patternis corrected by the correcting method of a photomask according to claim1, and a film thickness of the assist pattern after being corrected isthinner than a film thickness of the assist pattern before beingcorrected.